Companion Cube's Sacrifice

As an additional cost to cast Companion Cube's Sacrifice, sacrifice a creature.
Search your library or graveyard for a card
named Companion Cube, reveal it, and put it
into your hand. Then, if a creature you
controlled was sacrificed this turn,
you may return target creature card with
converted mana cost 3 or less from your
graveyard to your hand.
"Even in the depths of despair, the Companion Cube remains steadfast." - Aperture Science Announcement