See, I'm not doing this on purpose, it just keeps cropping up! First, an evil mage is the easiest build ever. Some notes:
1. You're going to lose Magnus, Gar (do not join evil characters) and Jayna (doesn't join mages). This leaves Virgil and Sogg (requires CH 9) as your
useful companions until the trip to Ashbury for Dog. GTA (level 12) frankly sucks - he has harm but I've never seen him cast it. The NPCs that you're going to get after the mines are also not stellar, except Z'an Al'urin, who's, well,
Al'urin.
2. The good news is that
Torian Kel is going to join you at 20.
3. Now, I do not define evil as "doing the most harm". I like doing whatever I want to do. However, if you help a person expecting a generous reward in the form of items, monies, blessings, fate points or plot progression, or if you resolve a conflict peacefully because you do not want blood on your brand-new Caladon Crusading Chain, the game might give you Good points. Thus, I take "Sold your soul", which grounds my alignment at -20.
Note: Sold Your Soul is in fact not necessary to maintain proper evil countenance. To lower your alignment, find a hobo (a shady character who asks for money) and do your best impression of Ayn Rand. you'll get -1 alignment per insult. Giving money to the hobo makes you more Good, +1 point per 10 bucks. Sold your soul just lets me see more game content in a single playthrough.
4. Being evil means manipulating people, which is making them react as you want them to. Sold your soul comes with a negative reaction adjustment, which is handy for those situations when you want to offend people, and the other end of the spectrum is covered by
Purity of Water and Charm (both 1st level spells). Buy a red suit, or, if playing a lady, travel to Razors Pointe from Dernholm (look it up) for that nifty dress. Stack three PoW on top of that and see Reaction rise to 200 - but watch out for bugs.
5. For the above reasons, Persuasion is good to have. Exploit blessings and don't forget to steal the ring of influence. Persuasion Mastery will make any NPC join you despite any disagreements they might have with you or other NPCs (except party size), and CH 20 will keep inactive characters waiting for you forever.
6. Intelligence helps to maintain a lot of spells, and doing that for a prolonged time requires Constitution. Which is why you'll do well to keep CN at 8 and set IN to 12 or so (whenever the third spell slot comes online). Plot requirements can be achieved with potions of IN.
7. WP powers your spells, namely Harm and Unlocking Cantrip.
8. Finally, conveyance is awesome - it has Teleport and Unlocking Cantrip, which you'll be using a lot. Take it.
So the Ultimate Build is going to look like this:
Sold your Soul, Dark Necromancy 2, Conveyance 2, Water 1, Mental 1, Persuasion 1, stats to power the whole deal (CH 9 upon arrival at Shrouded Hills and Persuasion 1 upon leaving Shrouded Hills). Then you gradually raise WP and CH to get Teleport by mid-20s and Persuasion Mastery by low 30s. Fire 1 also helps, given that you won't have charged rings, and Temporal is broken in a good way, so take it ;-) I also take Jolt for its wall exploit potential, but you can skip it and walk the dog instead (run away from a monster with Dog behind you getting free attacks on it).
Special note on invisibility:
Invisibility is cool. There's a ring of Invisibility in the game, but it's cursed and can only be used for
very short periods of time - so short that you'd better put the ring on hotkey to put it on and remove it, because going to Inventory will get you killed. However, I robbed Kree with this ring, powered the teleport back by staff and got a fate point next time I jumped the fence.