Kethis, the Hidden Hand is a mythic Legendary Creature — Elf Advisor card from Core Set 2020 Promos (PM20), collector number 211p. Current market price: $5.74 (foil $9.52), TCGplayer market prices via Scryfall.
Legendary spells you cast cost {1} less to cast. Exile two legendary cards from your graveyard: Until end of turn, each legendary card in your graveyard gains "You may play this card from your graveyard."