Hero spell cost.

The Warlord's ability belongs to the SPECIAL magic school. It has one mechanical feature: it requires mana but doesn't consume it. To make it consume mana, you can change the school to DESTRUCTIVE, for example, but there might be somewhat unpredictable results for heroes who have learned chaos magic.

Oh, that's interesting information; it seems the issue lies with the magic school!

If we assign Hellfire to chaos magic, we'll need to give it the sixth level of the Mage Guild – to prevent Hellfire from appearing in the guild.

And we definitely need to conduct tests because I've already experimented with a similar spell (Devil's Chain). When I assigned the chain to chaos magic, everything went haywire – the spell started consuming mana but stopped dealing damage!