Artifact Creature — Golem
At the beginning of your end step, Crystal Golem phases out. (While it's phased out, it's treated as though it doesn't exist. It phases in before you untap during your next untap step.)
"Once, a wealthy king commissioned an artificial creature to be crafted from the royal treasury, thinking it a clever ploy to have the jewels guard themselves."
—Azeworai, "The Golem's Wish"