At the beginning of each combat, double the power and toughness of each creature you control until end of turn.
"Should you hear witchstalkers howl,
Best to hide or run away.
They hunt witches vile and foul.
Fear both predator and prey."
—*To Walk the Wilds*
(24/09/2021)
If a creature's power is less than 0 when it's doubled, instead that creature gets -X/-0, where X is how much less than 0 its power is. For example, if an effect has given Bear Cub, a 2/2 creature, -4/-0 so that it's a -2/2 creature, doubling its power and toughness gives it -2/+2, and it becomes a -4/4 creature.
(24/09/2021)
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. Similarly, a creature whose toughness is doubled gets +0/+X, where X is its toughness as the effect begins to apply.
(24/09/2021)
If you control more than one Unnatural Growth, each one applies independently. For example, if you control two copies of Unnatural Growth, a 2/2 Bear Cub becomes a 4/4 creature when the first ability resolves and then becomes an 8/8 creature when the second one resolves.
