Blood Catalyst is one of the Skill nodes on the Rip Blood skill tree.
Node Description
Rip Blood no longer creates an orb of blood, and so can no longer return health, ward, or mana. If Rip Blood kills a target it is cast again for free at a near target near the dead one. This new cast can itself trigger an additional cast if it kills its target. This effect cannot trigger while you are out of mana.