Sliver Overlord

Most tribal payoffs reward you for already having the creatures; this one is a tutor that builds the deck around itself. Spend three generic mana and you fetch any Sliver straight into your hand: a lord when you want pressure, Crystalline Sliver when you need protection, Hibernation Sliver when you need an escape hatch, a singleton finisher when the board has stalled. That repeatable search is the engine, and it is why a five-color creature anchors a tribe that otherwise reads as a curve-out aggro deck. The second ability is the rarer kind of menace: gaining control of any Sliver, yours or an opponent's, with no upkeep tax and no clause that hands it back. In the mirror that theft swings the body count twice over, taking a creature off the other side while reinforcing your own, and over a long game the player with more mana to feed into activations slowly assembles the better board out of both libraries. As a 7/7 it also closes games on its own if the tutoring runs long, but the body was never the point: the point is a tribal commander that does not need a critical mass on the battlefield to function, because it manufactures that mass one card at a time.