Cheapest priced copy (Commander 2018): $9.49 at Card Kingdom, near mint, their price list as of .
Nylea's Colossus 
Enchantment Creature — Giant · 6/6 · Rare · Commander 2018 · #33
Constellation — Whenever this creature or another enchantment you control enters, double target creature's power and toughness until end of turn.
Legal inCommanderLegacyVintage
Not legal inBrawlStandardPioneerModernPauper
1 printing · $9.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Commander 2018 | 33 | Rare | $9.49 | — |
2 rulings from Wizards of the Coast
- 13 July 2018If 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.
- 13 July 2018If 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.