"Those who seek my protection receive it."
-Nylea, god of the hunt

Nylea's Colossus

Constellation — Whenever this creature or another enchantment you control enters, double target creature's power and toughness until end of turn.

Treasure Chest

"Those who seek my protection shall receive it."
—Nylea, god of the hunt

Commander 2018

Constellation — Whenever Nylea's Colossus or another enchantment enters the battlefield under your control, double target creature's power and toughness until end of turn.

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 Nylea’s Colossus -8/-0 so that it’s a -2/6 creature, doubling its power and toughness gives it -2/+6, and it will become a -4/12 until end of turn.

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. Toughness is doubled similarly.