Unleash Fury

• WWizards of the Coast2020-06-23
  Official Ruling
  If an effect instructs you to "double" a creature's power, that creature gets +X/+0, where X is its power as that effect begins to apply. If its power is negative, instead it gets -X/-0 where X is how far below 0 its power is. The value of X won't change if another effect alters the creature's power later in the turn.

Rulings via Scryfall