So what do you think happens to a wizard who successfully casts the Spell of Mastery?

It seems like he would become a demigod of some sort, at the very least. Presumably he controls all magic everywhere or something like that, which is a pretty significant amount of power. Or is he still mortal?

Just a fanciful question.

And here's a serious reply.

Such a wizard would either go mad with power, or get bored of it. If the former, expect a tyrant of the worst sort — even if all he had was Life magic, there'd be a Big Brother (in the form of angels and archangels) in every city, if not every single home. The best his subjects could hope for would be a massive suicidal revolt, eventually taking him out with the power of sheer numbers.

If the latter, he'd probably find a way to cast the Spell of Return on a former foe (the Spell of Recovery, perhaps?) just to give himself a reason to strive again. Or look and see if there was a third world beyond Arcanus and Myrror, and figure out how to reach it. Or eventually waste away, abdicating his throne out of sheer taedium vitae. Or perhaps father several powerful mages of his own; so that they could share the land, eventually begin to squabble, and start the whole cycle all over again....

I could see something like this. If there are two planes, why not more? It's interesting that, whatever the fabric of magic is in MoM, it extends to both planes. If it extends to more as-yet-undiscovered planes, the wizard might become aware of the existence of those planes after casting the SoM.

The spell description says "Caster obtains total control of all magic in both planes. Enemy wizards lose their ability to cast any spells and all summoned creatures come under your control." So the caster gains control of all magic on Arcanus and Myrror, which includes all of those summons that certain wizards like to have wandering around.

Your remaining rivals might not be banished to Limbo (yet), but they have been stripped of all ability to do any casting. The animation implies that something is broken for each rival, though they might not be banished. Seems like you might also gain essentially infinite casting skill (since you control ALL magic on the planes), which means that even if someone was poised with normal troops right on your doorstep and your capital was empty, you basically have the power to blow them to oblivion, so there really isn't any threat to you from any mundane source (which, again, all your former rivals now count as). Spell casting heroes might not be able to cast anything anymore either, unless you endow them with power. Of course, I've already sampled a wizard with no casting ability, so perhaps those rivals are more threatening than they might at first seem.

I'd imagine that the caster rules for awhile, but then starts looking towards further research and other planes/realms. Perhaps they start collecting spell books in all 5 colors. Then they start looking to maybe take direct control of the 5 realms that magic comes from. And from there, they start looking for other realms (hello, Dominaria!).

Or, as TwoHandedSword said, boredom sets in, and they effectively abdicate, either leaving the world effectively magic-less, or leaving it in a similar state to when the game started as they stop exerting control.

It belongs to the "Arcane" school, which doesn't have spell books but is a school of magic common to all wizards (along with other standard spells like Dispel Magic and Summoning Circle).

Given that the wizard is now more or less omnipotent (at least when it comes to magic)... well, what would they WANT to do? The answer is going to depend heavily on the personality of the wizard in question; even considering the implied personalities of the default wizards, an omnipotent Merlin is likely to behave differently from an omnipotent Tlaloc or an omnipotent Freya. How much any given wizard would actually USE their power is also an open question, since they aren't faced with an existential threat anymore and don't need to go all-out for the sake of survival.

I know it's a fanciful idea, but I'd enjoy living as the Master of Magic in the world I'd just conquered!

It would be an interesting way to start a MoM spinoff game.

What a wonderful idea!
New spell to research, "Interplanetary Teleport" and you've just discovered 4 more inhabited planets in your solar system!

Either that, or your wizard becomes one of the Masters in Eador: Genesis. That would be funny.

This is not true, it can be traded. I played on v1.40n with Horus on Extreme difficulty. I got kicked when I had two towns by neighbor Tauron and returned on a desert island with only one city. I had a second drawback when I fought 3 death knights with my heroes and lost, but killing 2 of them. I resurrected them thanks to life spell books but lost my magical items with flight. I was driven back so much that all other wizards researched the Spell of Mastery and started to spell them. I had to rush with my newly organized heroes from one wizard to the other to stop them. During the short peaces inbetween I traded spells and I got the Spell of Mastery, for an easy trade.

I do not save before entering a node or battle. I am using scouts for peeking, blocking roads and fake battles to drain enemies of their mana. I like hard games and I need the challenge to be defeated. I am losing 50 % on Extreme difficulty. I love these difficult games of the past.

Without challenge, I do not know what to do. If I was Master of Magic, I would probably recreate a smaller, artificial magic world and try to mimick my old glorious past, crafting smaller wizards that challenge each other in a sandbox world. If still bored, I would probably try to create a real opponent that possibly defeats me. Then that being would be my heritage to the world, and the philosophy of what to do would be my heritage to it. :p

Nice. I've never tried that before.

Insecticide might have done something to research weights. AIs are not supposed to offer their most research-expensive spell in trade. Violations can occur if you already have their most expensive spell, but they'll never offer the SoM in 1.31.