About the relationship btw buff skill gem "Archmage" and support gem "Fiery Death" in sorc.

"Archmage" increases spell cost of non-channeling spells about 5% of your maximum mana.

and "Fiery Death" buff gem have 60% chance to explode ignited enemies when they dies.

Now, when you used both gems simultaneously, every exploding dead enemy costs 5% of your maximum mana.
I think the game system is considering the every each explosion as "corpse explosion spell casted"

If you kill 10 enemies all at once and 10 dead corpses explode, about 60% of you maximum mana disappears.

I thinks it's not intended obviously.