This effect overwrites all creatures' colors. Creatures will be black instead of any other color or colors.

This effect overwrites all lands' land types. Lands will be Swamps instead of any other land types. This won't affect a land's supertypes (such as basic or legendary). Lands will lose their normal abilities and have the ability ": Add to your mana pool" instead.

If a permanent is both a creature and a land, Nightcreep replaces all of its land types with Swamp, but it doesn't affect its creature types. The creature land loses all abilities generated by its rules text and its previous land types but retains any abilities granted to it by other effects. For example, an animated Blinkmoth Nexus becomes a black creature land with the land subtype Swamp whose only abilities are flying and ": Add to your mana pool."

A land or creature that enters the battlefield after Nightcreep resolves won't be affected by it.