Demon Slayer is a melee attack ground hero with Executioner mercenaries. His skill is an AoE dmg / Healing with status effect detriments to targets hit.
Demon Slayer's divine skill pulls far enemy heroes into melee range and stuns them. He is very useful for countering Ambrosia's defensive fortitude as well as certain Battle Royale bosses and the Ether Ogre Guild Boss.
The Holy Hammer is the best equipment to use with Demon Slayer as it boosts his Executioner mercenaries and also allows his divine to do significantly more DMG.