Talk:088 - Grimer/@comment-77.5.215.151-20160727013143

Grimer and Muk can hold the item "Black sludge".

Held by a Poison-type, it restores 1/16 of its maximum HP each turn. Held by a non-Poison-type, it drains 1/16 of its maximum HP each turn.

http://pokemonrevolutiononlineguide.jimdo.com/item-guide/

You can't get it with Covet (attack)! Just catch the Pokémon.