Vivien, Monsters' Advocate is a mythic Legendary Planeswalker — Vivien card from Ikoria: Lair of Behemoths (IKO), collector number 175. Current market price: $3.22 (foil $3.15), TCGplayer market prices via Scryfall.
You may look at the top card of your library any time. You may cast creature spells from the top of your library. +1: Create a 3/3 green Beast creature token. Put your choice of a reach counter, a vigilance counter, or a trample counter on it. −2: When you next cast a creature spell this turn, search your library for a creature card with lesser mana value, put it onto the battlefield, then shuffle.