A ubiquitous and woefully underused spell, present in all of the Final Fantasy games released in America. Drain is available to black mages in FFII, through the esper Ifrit in FFIII, and through the materia "HP Absorb" in FFVII. It can probably be found lying around in FFVIII, but I haven't played it that far yet.

The effect of drain, is, of course, to take hit points away from the enemy and transfer them to you. Note that the conduit reverses if you attempt to use this spell on the undead. You wind up taking some of their negative hit points, which has the result of doing you damage. The drain spell is actually rather weak, and ceases to be useful shortly after you learn it in any case.

A related spell, known as "osmose" in FFIII and available through the "MP Absorb" materia in FFVII, takes magic points from the enemy instead, and could be considered a "brain drain".