"In magical battles, the conservation of mana is everything! He who can cast the last spell will be victorious."Power Drain is a second-level order magic spell in Heroes of Might and Magic IV that costs three spell points and causes the target's spells to cost double the normal spell points.
Virtually useless against magic heroes of even a modest level unless combat runs for many rounds or you have caught them low on spell points, it is still capable of stripping much of the effectiveness from spell-casting creature stacks, who are equipped with a (usually mediocre) set number of spell points no matter how large the stack. Use it to curtail Fireballs from faerie dragons and stop Leprechaun and Angel spell-casting cold.