Altar of Mogis is a common Artifact card from Battle the Horde (TBTH), collector number 11. Current market price: $3.62, TCGplayer market prices via Scryfall.
At the beginning of the Horde's first main phase, reveal an additional card from the top of the Horde's library. The Horde casts that card. Hero's Reward — When Altar of Mogis is put into a graveyard from anywhere, the Horde sacrifices two Minotaurs.