Some Tyranids are able to internally energise a form of bio-plasma that they vomit forth as a ball or bolt of blinding green fire. The plasmic material is held in their gut and energised by rasping plates in their oesophagus. This process makes a high-pitched screaming which precedes a bio-plasma attack.
Creatures that use bio-plasma can make a single extra close combat Attack, worked out at double the model's Initiative (ie, Initiative 8 for a Gargoyle, 4 for a Carnifex, etc). Bio-plasma can also be used by models that are within 2" of the enemy as well as by models that are in base contact.
Bio-plasma hits on a roll of 4+ on a D6 regardless of the target's type or Weapon Skill. It inflicts a single hit with a Strength equal to the creature +1 (maximum of Strength 10). Normal armour saves are allowed. Remove models as you would other close combat casualties.
After bio-plasma attacks have been resolved, Tyranid creatures make their other close combat attacks in sequence with their normal Initiative value. Note that since bio-plasma kills count as close combat casualties, a creature who starts the combat in base contact will get its full number of Attacks (including benefits from close combat bio-weapons), even if the model it is in base contact with is removed by bio-plasma.
Errata — Chapter Approved: Codex: Tyranids Q&A (v2.0): Bio-plasma always permits the target a normal Armour save, regardless of whether the attacker is a monstrous creature — it's an enhancement/bonus attack before the creature brings its full strength to bear, not subject to a monstrous creature's usual armour-ignoring close combat rule.