In this build, for some reason, Hellfire and Devil's Chain deal an incredibly high amount of damage, which is most noticeable in the original campaign, but also on other downloaded maps where these spells are present, making the game very difficult.

It turns out that this is due to the "Ice and Fire" map, which modifies them for hero balance. Of course, it's clear that you need to delete all other maps before playing, even if the launcher complains about it.