Demonologist

From the Might and Magic Wiki - Might and Magic, Heroes of Might and Magic, King's Bounty

The Demonologist is a specialist in both Death Magic and Nature magic.

This unlikely union produces an unusual result - the Demonologist receives five unique spells, one for each of the lesser of their Demonology and Nature Magic skill levels. All of the spells are Summon spells, and each produces non-undead Death Units. What makes these spells special (besides the condition for acquiring them) is that they produce much larger numbers of units than corresponding Nature spells would, though at a significantly greater mana cost than normal spells of their level. Because of the power of these unique spells, Demonologist is a fantastic hero class. Furthermore, once these spells are learned (they are acquired automatically upon achieving the necessary skill levels), you do not lose these spells even if you change specialization or go Archmage, though after doing so you won't learn any more even if your skill levels increase. In the HoMM4 campaigns, Gauldoth Half-Dead is predisposed to becoming a Demonologist.

The unique spells (by level) are:

Basic: Summon Imps (costs 5 mana)

Advanced: Summon Cerberi (costs 8 mana)

Expert: Summon Ice Demon (costs 12 mana)

Master: Summon Venom Spawn (costs 18 mana)

Grandmaster: Summon Devils (costs 18 mana)

Stating it again, the highest level of Demon-summoning spell your hero will know is equal to the lesser of his Demonology skill and Nature magic skill (thus, both must be at Grandmaster level to learn