Skaven weapons contain enough warpstone to count as magical, and so can damage creatures invulnerable to non-magical weapons. Attacks from weapons with this special rule count as magical.

Assassins' and Gutter Runners' poisoned weapons, plague censers' gas and hits, Warlock Engineers warp-blades, warplock pistols, poisoned wind globes, Jezzails, Ratling Guns,
Warpfire Throwers and Warp-lightning Cannons.