Deekah, Fractal Theorist is a rare Legendary Creature — Human Wizard card from The List (PLST), collector number C21-26.
Magecraft — Whenever you cast or copy an instant or sorcery spell, create a 0/0 green and blue Fractal creature token. Put X +1/+1 counters on it, where X is that spell's mana value. {3}{U}: Target creature token can't be blocked this turn.