Sefris of the Hidden Ways is a mythic Legendary Creature — Human Wizard card from Secret Lair Drop (SLD), collector number 2508. Current market price: $4.75 (foil $5.85), TCGplayer market prices via Scryfall.
Whenever one or more creature cards are put into your graveyard from anywhere, venture into the dungeon. This ability triggers only once each turn. (To venture into the dungeon, enter the first room or advance to the next room.) Create Undead — Whenever you complete a dungeon, return target creature card from your graveyard to the battlefield.