"This will make that sorcerer think twice about casting Lightning Bolt!"Magic Leech is a third-level Death Magic spell in Heroes of Might and Magic IV that costs five spell points and causes the target to pay double the normal mana cost for all spells they cast, and furthermore, the caster of Magic Leech receives back one mana point per two mana the target spends.
Similar to, but better than Power Drain, the extra mana is not wasted - it recharges the Leech caster's supply!
This extra cost this spell imposes will probably not prevent a spellcaster hero from casting (unless you catch them short on mana), but can slow or shut down spellcasting creatures. Single-cast creatures like angels and nightmares will not be able to cast at all, while spellcaster creatures, like genies and faerie dragons will run out of mana much earlier in the battle.
Imps have a related power, called "Mana Leech", as an ability.