Gamekeeper

The deck's namesake and heart: when it dies, exile cards from the top of your library until you exile a creature, then put that creature into play. Sacrificing it to a free outlet cheats an otherwise-uncastable threat directly onto the battlefield, and the death trigger doesn't care how big the creature is.

The redundant combo half. When an opponent controls more creatures than you, on your upkeep you may dig through your library until a creature enters play. It turns the deck's lone Gamekeeper or fatty into a free cheat-into-play without even needing a sacrifice outlet, giving the deck two independent routes to the same payoff.

A free sacrifice outlet that fires Gamekeeper's death trigger at instant speed while milling. It doubles as a kill condition: sacrifice the giant creature you just cheated into play to mill the opponent for its power, often decking them in a turn or two.

The deck's assembly engine. Fetch three copies of a combo piece to guarantee one resolves, or grab a reanimation spell plus a target so whatever gets milled or exiled comes straight back. It converts a generic hand into the exact pieces the combo turn needs.

Tears the key answer out of the opponent's hand before you combo, and its flashback is paid by sacrificing a creature, which also triggers Gamekeeper's death ability. One Therapy can strip two cards while advancing the deck's own sacrifice plan, making it both protection and enabler.

A turn-one play that pre-emptively clears the counterspell, removal, or graveyard hate that would otherwise break up the combo. Against control and combo it buys the window the deck needs to assemble and resolve Gamekeeper or Oath of Druids unanswered.