Hero spell cost.

 

So, is the issue with Kha-Belekh? Does the character not consume mana? And is there a way to change this?

The details aren't clear here; there aren't many mods with this ability, so it's hard to say for sure what the problem is.

The problem shouldn't be with the hero. In general, it's a strange ability... Does Hellfire Wave initially cost 5 mana points? In the "Invasion from the Underworld" campaign, Hellfire Wave costs 0 mana points; this might be related to this bug.