Test as soon as the kaven sorcerer uses one or more pieces of warpstone to power his spell. Roll a D6. If the result is more than the number of warp tokens used then the Saven is not affected by it and the spell may be cast as normal. If the result is the same as or less than the number of warp tokens used the Skaven sorcerer has failed to control the tide of Dark Magic. The spell is not cast, and the Skaven's body writhes and mutates into a Chaos Spawn, a foul blob-like monster which sprouts tentacles and pincers. The Chaos Spawn is insane and almost mindless, and hauls its twisted body about in confusion, destroying anything it touches.

As soon as the Skaven turns into a Chaos Spawn it makes a random move 2D6". Use the scatter dice to determine which direction it moves in. From then on the Chaos Spawn moves 2D6" randomly during its own movement phase. As this is compulsory movement the Chaos Spawn is moved before other troops.

A Chaos Spawn will move over any troops in its way, striking them with its pincers, or covering them with corrosive slime as it does so. These attacks are worked out as the Chaos Spawn moves over its victims in the movement phase and so are an exception to the normal turn sequence. Each model moved over is automatically hit once. A Chaos Spawn does not engage in hand to hand fighting as such, but any enemy models in base-to-base contact during the hand to hand combat phase can strike blows against it as normal. The Chaos Spawn does not strike back in the hand to hand combat phase.

Neither the Chaos Spawn nor its opponents are obliged to remain and fight models they are touching, but can move off as they wish (the Chaos Spawn itself must move and moves randomly as described). The Chaos Spawn is almost completely mindless and does not ever take leadership based tests. It cannot be affected by psychology at all, and it does not take break tests and so cannot be broken.