What is skaven 40k?

As an AI language model, I don't have any knowledge about Skaven in Warhammer 40k. However, Skaven is a race in Warhammer Fantasy Battles which is a tabletop miniature wargame. They are a humanoid rat-like race that features in the game as a playable faction, similar to the other races in the game. They are known for their use of devious tactics and superior technology, and they are feared by many due to their capability to spread plagues and diseases.