This Rune has the power to drain the opponents health and recover your own. Sounds like vampire, doesn't it? Actually this works with physical weapon damage only, so it's useless for mages and supporters.
The gain of this rune depents strongly on the game it appears in. While in Suikoden II, III and Tactics only critical hits restores 1/3 of the damage dealt to yourself, you will most likely combine it with a Killer Rune for noticable benefits. Suikoden IV increases the drained health to the full amount of damage inflicted by criticals, making the Drain Rune even more worth a rune slot. Its effect in Suikoden V is the best, when it spends health for every physical damage dealt, not only for criticals.
Lorelai brings a Drain Rune during her performance in Suikoden II, and creepy winger Landis always has one in Suikoden III. - Celes