Nightcreep — ruling 1

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

Nightcreep — ruling 2

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 "{T}: Add {B}" instead.

Nightcreep — ruling 3

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 "{T}: Add {B}."

Nightcreep — ruling 4

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

Nightcreep : règles, fonctionnement et interactions (MTG)

Nightcreep is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Until end of turn, all creatures become black and all lands become Swamps.

How it works : Nightcreep

Original English text — Wizards of the Coast / Scryfall

#1May 1, 2006· Wizards of the Coast
This effect overwrites all creatures' colors. Creatures will be black instead of any other color or colors.
No other card is named in this ruling.
#2May 1, 2006· Wizards of the Coast
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 " instead.
No other card is named in this ruling.
#3May 1, 2006· Wizards of the Coast
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 ."
No other card is named in this ruling.
#4May 1, 2006· Wizards of the Coast
A land or creature that enters after Nightcreep resolves won't be affected by it.
No other card is named in this ruling.