As Mazdamundi, they will often be your second target, after Skeggi, as this faction starts with ownership of the Mirror Pools of Tepok, which contains a ritual resource site. The natural quest chain for Mazdamundi often includes taking Mirror Pools of Tepok for a reward; Skaven corruption will also spread into the adjacent Isthmus of Lustria, which is Mazdamundi's home province.
The faction starts with two cities in Forests of the Viper and may quickly colonize the third as it is in ruins. Two armies will likely be required to defeat the fortifications built at Mirror Pools of Tepok as the AI army will likely stay safely behind the walls. Sometimes, Clan Gnaw will fight Ss'ildra Tor (Dark Elves) which will make it easier to conquer Forests of the Viper, but ensure you wipe them out, otherwise they will continue to spread Skaven corruption to nearby provinces.
Although Clan Gnaw is based in the Badlands in the lore, they are on the other side of the world in Lustria in the Total War: Warhammer games.