Drain is an ability that causes the attacking unit to restore (similar to heal but with some difference) the equal amount of health as the final damage(s) to defending unit(s). It is usually underrated and underused, but has some potential to be a very powerful ability.
Drain can be partnered in several ways. Attack Plus directly increases the damage to be restored, Slayer can increase that amount significantly, while Whirlwind will cause more units to be attacked, restoring even more health. Besides, a very rare case of double Drain (Equipping Soul Leech to a drain unit) will cause the double health to be restored from the final damage.
Drain is able to restore health of an unit even if it is wounded.