Maîtres du Marais

Land cards not on the battlefield aren't affected by Swampbenders's second ability.

Swampbenders's second ability causes each land on the battlefield you control to have the land type Swamp in addition to its other types. Any land that's a Swamp has the ability "{T}: Add {B}." Nothing else changes about those lands, including their names, other subtypes, other abilities, and whether they're legendary, basic, or snow.