The Endstone is a mythic Legendary Artifact card from Edge of Eternities Promos (PEOE), collector number 240p. Current market price: $2.67 (foil $2.72), TCGplayer market prices via Scryfall.
Whenever you play a land or cast a spell, draw a card. At the beginning of your end step, your life total becomes half your starting life total, rounded up.