[quote=@TheUnknowable]
On this world however, the magic is subject to incursions from other worlds which constantly disrupt it.  Instead of rivers, you get puddles, or tiny streams, maybe even weak whirlpools.
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On the other hand you might also get a sudden surge, changing a simple Ember spell intended to light a campfire into an unstoppable torrent of flames which burns down all our tents.

[quote=@TheUnknowable]
Casting large-scale spells would require that you create an artificial pool of energy to tap into by tying multiple sources together.  These sources could be location based, or be tied to objects/herbs/ingredients/etc.

If we went with the above description it would limit his powers to whatever sources he could find, keeping him from going OP despite the technical fact that he's insanely powerful.  Would it work with your character idea, though?
[/quote]
Depending on how long he's been on Mym he may also have learnt to harness a store of energy in things like focusing crystals. These would allow him to have instantaneous access to high-level spells when needed, but would basically be one-shot, irreplaceable items which couldn't be recovered within the scope of the adventure.

The most likely use for these would be as a catalyst for summoning powerful thralls or animating corpses for a short period, but once the power had burned off the minions would fall apart once more.