Ugin's Nexus is a mythic Legendary Artifact card from Khans of Tarkir (KTK), collector number 227. Current market price: $1.70 (foil $8.53), TCGplayer market prices via Scryfall.
If a player would begin an extra turn, that player skips that turn instead. If Ugin's Nexus would be put into a graveyard from the battlefield, instead exile it and take an extra turn after this one.