What are your favorite custom spells in the Elder Scrolls games? (Make sure you specify which game you're referring to.)

I'll start with some from Arena:

Ultimate Shield:
Shield 1 + 99/level
This gives you a powerful shield spell that will absorb all the damage you take (including things like poison damage). The spell has no finite duration, so you can cast it in town before traveling to the dungeon. If you can't cast it, you can make a weaker version, or you can Fortify Intelligence and use Potions of Restore Power to recharge your spell points.

Absorption:
Fortify Willpower to 100, decay rate 10/round
Absorb Spells 1 + 1/level (IIRC)
It turns out that your Willpower stat affects how often you absorb spells, so at high Willpower, even a minimum strength Absorb Spells is enough to absorb enemy spells a decent percent of the time.
If you're a Sorcerer, remove the Absorb Spells component and you will find this spell to be very useful. Optionally, add a Fortify Intelligence effect to benefit more from the absorption.

Ultimate Unlock:
Open 1 + 99/level
This spell may seem like overkill, but it does one thing that lesser Open spells don't; it will unlock doors in main quest dungeons that are supposed to require special keys to open, provided that your level is high enough. Some main quest dungeons can be completed quickly this way. (One, in particular, requires getting 2 keys from different parts of the dungeon, and that's the majority of the dungeon. With this spell, you can skip that.)

Designate as Non-Target, all maxed out:
I consider this an interesting case, as the cost increases so fast that it can overflow the cost twice. In version 1.06 (haven't tried 1.07 yet), this will crash the game in the spellmaker. (Anyone know why this causes a crash?) Of course, actually using this spell is impractical because you can't actually make it in the first place.

How come no one is posting in this topic?

Anyway, I found a possibly useful spell for Morrowind, provided you don't mind exploiting a certain bug (one that is annoying when it happens to you, so perhaps it's fair):

Drain Strength 100pts for 1 second on Target
Fortify Strength 100pts for 1 second on Target

This spell (which is a lot cheaper than Damage Strength) works as follows:
1, Target's Strength goes down 100 points
2. Target's Strength goes up 100 points
then, one second later:
3. Drain wears off, target would regain Strength, except that Drain wearing off will not raise a stat above its maximum
4. Fortify wears off, lowering the stat 100 points. This is permanent until healed with Restore Strength.

The result is that the enemy now has 0 strength permanently, and if the enemy is carrying anything, the enemy will not be able to move (but can still attack or cast spells).

This can be applied to other stats as well, but note that multiple hits with this sort of spell do not stack, so a high magnitude is necessary for this to be useful. Also, note that order matters: Fortify than Drain has no net effect here.

Maybe because ES magic systems in general are dull and clunky, and in case of custom spells particularly so?

From what I remember of Morrowind, you had to know the spells you wanted to combine, which was rather dumb since it wasn't even your character who did the mixing, you had to pay someone else to do it for you.
Also, there was no way to organize, hide, or delete spells, so you had to wade through a lot of useless stuff. This, and the fact custom spells cost more mana than a standard spells of the exact same type and power was a deterrent to learning and experimenting with new spell combos.

But most importantly, the vast majority of enemies in Morrowind is so weak and squishy that anything beside destruction spells was fundamentally useless. Why bother with crowd control and debuffs, when you could just kill everything for much less mana and in much less time?

Beside some basic alteration spells I made for traveling and utility purposes (jump, levitate, and open locks mostly), I don't remember finding much use for custom spells.

Because magic isn't just for combat. Consider one of my newest spells, Fortify Mercantile 400 points for 1 second on self (created by repeating the 100 point effect 4 times in the spell). Useless in combat, but very handy when going shopping: I can now sell cheap items for all of the merchant's money. Or how about Fortify Alchemy or Enchant? (I actually consider Restoration to be my favorite magic school in Morrowind so far.)

Also, Mysticism (Absorb Health) can be used to damage enemies *and* heal yourself at the same time, plus it is harmless if it gets reflected.

There's also the fact that certain spells are *fun*, even if they might not be practical in normal gameplay.

By the way, if you want more Magicka to play around with spells, here is a spell that will give you plenty (who needs Restore Magicka effects?):
Drain Intelligence (exactly enough to lower it to 1) for 1 second on self
Fortify Magicka (ideally 100 points, but lower values work) for 1 second on self

One cast of this spell will result in you gaining a huge amount of Magicka, allowing you to use other spells as much as you want, and when you run low (or even before then, since the spell stacks with itself), just cast the spell again.

Well you sort of keep answering yourself to be honest. See, most TES fans don't even enjoy powergaming in Elder Scrolls games given how dull they become when you do, let alone abuse exploits. I made a bunch of utility custom spells in my TES 'career', especially useful was AOE unlock, but most turned out to be either utterly useless or broke the game in some respect - which is why I stopped making use of the mechanic as the primary reason for me to play TES games is roleplaying and immersion, not glitch hunting.

Now I'm not saying you're wrong to enjoy the game in this fashion - after all, the whole point of TES games is that they allow you to do whatever you want. Most people that I know of quite simply do not enjoy playing the game in that manner tho.

Thing is, the spellmaker is actually the thing that got me interested in the series in the first place. There are so many things you can do with the spellmaker, and I think it would be quite interesting to see what other players have come up with.

I am currently trying to make a "Jump across the entire continent" spell, and have currently gotten half way there (Jump 100, Fortify Strength 100 (encumbrance issues), Fortify Acrobatics 600 + I now have Boots of Blinding Speed).

In a sense, part of spellmaking is just to see what interesting things you can do. Some of the so-called "powergaming" is there so that I can do more interesting things (spellcasting is more fun if you don't have to worry about Magicka or spell failure).

Also, if you are looking for a challenge, the way I see it, this is the wrong series.

It would, of course, help if the spellmakers in these games were better balanced, and the known spellbook spells were close to optimal from a powergaming perspective (and the game balanced around them), so that you didn't have to worry about breaking the game by using the spellmaker. (Some balance problems are quite blatant, like with Daggerfall's Regenerate Health effect; the per-level component of the magnitude is much cheaper than it should have been.)

Edit: Another example of poor balance is the Powerwell spell in Morrowind, which is useless as written. Fortify Magicka effects could be useful if you need just a little extra Magicka to cast that one spell you need, but not when the spell costs more than the Magicka it provides.

While I'm not a "power gamer" myself, I admire dtgreene for his creativity with using the spellmaker or finding exploits. It's not the way I play, but it's a very legitimate way. You don't play the game, you play with the game, explore its potential.
Yes, Morrowind, the base game, becomes quite (too) easy once you have survived the first couple of hours, but then you don't have to follow the trodden paths. You can invent and follow self-imposed rules, or you achieve your goals in different ways, ...
A role player would choose a role which makes playing more interesting (I defeated Dagoth Ur without weapons, without armour, with only healing spells, or I played the game as a pacifist, without fighting, both very interesting experiences), a power gamer would try to find what the game let's him do, like inventing all sorts of spells (why fight with a strong enemy in the normal fashion, if you can take away his strength or health?, why should you use the silt strider or propylon index system, if you can jump around the world? ...). He would try to overcome game limitations (like the maximum skill level of 100), or do a speed run (kill Dagoth Ur within one or two minutes after being released in Seyda Neen - it's possible without cheating), etc.
Not for everyone, but legitimate, and I can understand that it's entertaining - and, most of all, a very creative way of playing.
And it's also worth noting that Morrowind is one of a very small number of games which you can enjoy for a very long time without ever touching the main quest (which isn't saying that the main quest is boring)!

Here's a few (non-optimal) Morrowind custom spells I think are interesting:

1. Touch of Death:
Drain Health X points for 1 second on Touch
This is the closest you get to an Instant Death spell in Morrowind. Its advantage over damage spells is that it kills the target instantly, whereas damage spells, even those with a 1 second duration, do damage over time. The disadvantage, of course, is that an enemy with more than X health will be unaffected by this spell. (Of note, Cliff Racers have 45 HP, so if you want to design a spell to kill them specifically, that's the magnitude you should use.) Essentially, this gives you a spell similar to D&D's Power Word, Kill spell. (Apparently, this effect is really cheap in Oblivion, making this sort of spell really good at low levels before enemies start getting over 100 HP.)

2. Delayed Heal:
Drain Health X points for Y seconds on Self
Restore Health X points for 1 second on Self
Here's an interesting one: When you cast it, you will lose some health and then immediately regain it. Then, after Y seconds, you will be instantly healed X points. This can be useful if this happens in the middle of combat, but is significantly more expensive than a standard healing spell. You can substitute Absorb for Restore (but note that if you use Target and you miss, the Drain Health effect fails to end and you don't get your health back). Another idea is to use a constant effect item for Drain and heal yourself some other way; this way, if you need instant healing, all you need to do is unequip the item and you are healed. This trick can be done with Fatigue or Magicka as well.

Note that Oblivion changed the way Drain effects work, so this trick won't work in that game.

3. Blind Accuracy:
Blind X points for Y seconds on Self
This spell (assuming you aren't using the component of the Morrowind Code Patch that fixes Blind) will reduce your visibility, but will actually increase your accuracy. Of course, you are better off fortifying your weapon skill (or, if you have earned a specific spell, your Attack). On the other hand, if you are weak against Magicka, this can be a more cost-effective way of increasing your accuracy.

4. Spell Exposure:
Weakness to Magicka 100 points for 1 second on Self:
The idea here is that you cast this spell and then equip an item with a Drain Health/Fatigue/Magicka effect in a safe place. After doing this, restore your Health/Fatigue/Magicka normally. Now, when you are in a dangerous place (like the middle of combat), unequip the item. This will instantly restore the resource in question, allowing you to continue fighting.

By the way, one instance where you might want non-damaging offensive spells is if you are using the mod that makes 2 Cliff Racers appear when you kill one. With that mod, killing them isn't a viable option; you need to disable them and then run away.

That goes without saying, but for any other purposes I just found the standard spells to be more than adequate. Of course, I had fun playing Superman in the skies of Morrowind just as everyone else, but that was mostly for the lulz. For traveling I preferred to rely on teleport spells and occasionally a 10 seconds levitation+speed. Me? I would have killed for some kind of innate mana regen mechanic or meditation spell (e.g. the player falls into a trance, restoring his own mana over time, but cannot perform any other action while the spell is active). Of course, that wasn't possible because while a restore magicka spell exists, nobody teaches it in vanilla Morrowind; and even if someone did, you cannot channel spells in Morrowind anyway. This reminds me how I used to be very annoyed by all the low level lemmings who kept throwing themselves at me, until I found a little exploit to handle them peacefully: Calm Humanoid immediately followed by intimidate and then admire to raise their disposition to a decent level. I *think* that was an exploit at least, because even if they stopped fighting, they kept yelling at me.

Interestingly enough, if you have Tribunal installed, there is a "spell" with ID "regen_imperfect". If you use the console to add the spell to your character, you get permanent health and magicka regen (only 1 point, however). You might want to try this and see how it feels.

Is there anyway to get a castable Restore Magicka spell without mods (via the console, of course) so that Restore Magicka spells and items can be made?

Edit: One question, for your levitation + speed spell, how much speed?

I haven't touched Morrowind in like 10 years, so maybe I'm mistaken about restore magicka being an actual spell. I thought it was, but the wiki says that's not the case. I guess I just looked at the regen mana potion effect and assumed there had to be a castable version somewhere in the game files. Sorry about that. :)

I don't remember exactly, but I think I used low levitation strenght + high speed because it was more efficent than just high levitation strenght. It was quick though. Enough to cross half Vivec while it lasted.

Here is one that can be quite useful: Minor Heal

For Morrowind:
Restore Health 2 points for 2 seconds.

What we have here is a spell that restores 4 health during its duration. It costs only 1 Magicka, and is therefore as easy as possible to cast. Since spells take 1 second to cast and have a 1 second cooldown, you can cast this spell repeatedly without any of the effect wasted. Great for leveling up Restoration skill, which in turn lets you cast powerful Fortify Attribute/Skill spells more reliably.

For Arena, it's something like:
Heal Health 1-2 + 1-2/2 levels

The idea is that the game rounds the cost down, so this spell is cheap to cost. In fact, at level 11 (6 for Healers, 21 for Battlemages), the cost becomes zero (0). This means that you can cast the spell to heal yourself at no cost. I recommend creating a Fatigue version of this spell which becomes free to cast at level 6; great for resting without having to use the Rest command.

For Daggerfall, try something like:
Restore Health 1-2 + 1/2/level
Restore Health 1-2 + 1/2/level
Restore Health 1-2 + 1/2/level
(Yes, the same effect 3 times.)

In Daggerfall, the trick is less effective because every spell must cost at least 5 spell points. However, by including the same effect 3 times, we get a spell that levels up Restoration 3 times as fast. Similar tricks work for other schools of magic. (A weak Damage or Absorb Health can be a good attack spell for practice.)

I am playing Daggerfall at the moment - any spell hints?

I am going to try several of the ones you use in MW.

The most gamebreaking spell effect in Daggerfall is Regenerate Health. Seriously, make a spell of Regenerate Health 1 + 25/level per round and it is far cheaper than it ought to be. Both Regenerate 25 + 1/level per round and Restore Health 1 + 25/level are significantly more expensive.

For offense, I found Continuous Damage to be quite effective, though it does use up your Magicka fairly quickly. On the other hand, one cast is enough. Unless you are playing a Sorcerer (or took Absorb Spells at character creation), choose the area centered on self option. Area at range is tricky to aim and can hurt you (or be absorbed) if you're not careful. Target at range is too hard to aim.

It is a good idea to have a Slowfall spell handy just in case you fall into the void. If this happens, cast Slowfall to prevent falling damage, then press Alt-F11 to return to the dungeon proper. If you actually want to explore the void, you will, of course, need a Levitate spell; just cast your Slowfall spell first. (Levitate doesn't prevent falling damage when cast in mid-air.) Creating your spells with shorter durations will give you more opportunities to cast the spell, giving you more experience.

One nice way to save a bit of money is to cast a Fortify Personality spell and then talk to the shopkeeper. The duration isn't important, and the magnitude need only be high enough to raise the skill to 100.

Not all effects are useful. Charm appears to be useless (it doesn't actually work on non-hostile NPCs in this game), and Soultrap isn't needed in this game (you don't need souls to enchant items).