The Overlord is in NHF_E (in the extended version). And it is permissible for him to be overpowered, both in essence and in mechanics.
Added 43 seconds later
If the enemy is a demon, Hellfire Wave does not deal damage.
Added 3 minutes later
If the Overlord has non-demons in his army, they will die.
Added 5 minutes later
Considering all of the above and the demon's weak magic, the formula "50+50*SP" would be more appropriate, instead of "250+0*SP" as it is now.
Well, considering that he is in NHF_E, I suppose I agree – it needs to be stronger than what I suggested.
Блистает там воздушностью прогалин.
Но между кедров, полных тишиной,
Расщелина по склону ниспадала.
О, никогда под бледною луной
Так пышен не был тот уют лесной...