@Slam I'm assuming you liked it because you want the spoiler?
Spoiler!
Basically his mana is too dense for the world, and until he learns to use his mana with no leakage, every time he uses his mana free-style, as opposed to using a skill (e.g. Flame Arrow), he adds to the mana density of the world. The world in its current form can only support up to rank 4 mana, and not turn hostile to normal plants (and animals and people too I think?). Every quest he completes fixes a small percentage of the mana density of the world. He later learns how to avoid the apocalypse via some mana recycling method he uses with Ellie... only to have some POS force combat in such a way that tips the world over the edge, and then they have to set about fixing the result.