Does 40 damage plus 10 more damage for each Energy in the Defending Pokémon's Retreat Cost (after applying effects to the Retreat Cost).
Muk ex is one of the most prominent cards in the unlimited format of Pokémon Trading Card Game play. Grimer's Ascension attack, which allows the player to search for a card that evolves from Grimer and put it onto Grimer, ensures that a player can get a Muk ex in play on their first turn. Muk ex's Toxic Gas Poké-Body is extremely useful in unlimited because it cancels out Crobat G's Flash Bite Poké-Power, along with a number of other Poké-Powers that are used in combination with Trainer cards such as Scoop Up and Team Galactic's Invention G-105 Poké Turn to cause extreme damage on the first turn. The aforementioned cards are parts of one of the most popular unlimited format decks which Muk ex seeks to counter.
This card's Toxic Gas Poké-Body comes from the Pokémon Power of the same name possessed by FossilMuk, which had a very similar effect. The primary difference is that Muk ex must be active for the effect to work, whereas Fossil Muk's worked even from the Bench.