If a Tyranid Swarm Wyrm comes into base to base contact with any enemy ship during the Tyranid player's movement phase it automatically detonates against the armour value that it impacted on (ie: if a swarm Wyrm comes into contact with a cruiser's port side, the damage would resolve on the port armour). To resolve the detonation simply roll 2D6 against the target's armour (ignoring shields) for each Wyrm. Holofields may save as normal. After the attack has been resolved replace the Swarm Wrym with a blast marker (which should also be in base to base contact with the enemy ship), regardless of whether or not the attacks were successful. For every three points of damage done to a target ship by Swarm Wyrms, that target receives one automatic fire critical hit to represent excess amounts of acid burning through the hull. This can be "repaired" (extinguished) as normal.

Because the Wyrms are so small, the opposing player has the option to fire at the Swarm Wyrms with your defence turrets if they wish. Simply roll a D6 per turret, for each 5+ a Swarm Wyrm is detonated before it can make its attack. Replace destroyed Wyrms with blast markers as normal. Note that if an enemy ship uses its turrets to fire at Swarm Wyrms, it may not use them to defend against attack craft or torpedoes in following ordnance phase.

Swarm Wyrms are self-contained organisms and do not carry any smaller Tyranids. Because of this, any vessel which makes a hit and run attack against a Swarm Wrym receives a +1 modifier. Swarm Wryms may not participate in any boarding actions. If an enemy player attempts to ram or board a Swarm Wyrm, it will automatically detonate. Otherwise, Swarm Wyrms can not be detonated during the enemy player's turn.

Swarm Wyrms must be fielded in squadrons (swarms) of 6 to 18. For scenario purposes, Swarm Wyrms do not have any assault points.

Swarm Wyrms, being the simple creatures that they are, have a condensed instinctive orders table shown here: