Kethek, Furnace Stoker

At the beginning of each player's upkeep, they
may exile target artifact. (The turn player chooses the target.)
Whenever an artifact you control leaves the battlefield, you may put a +1/+1 counter on Kethek, Furnace Stoker.
Kethek, Furnace Stoker has hexproof as long as there are five or more artifact cards in exile.