Bio Plasma
From Exterminatus Manual
Bio-Plasma is the medium range weapon of the Tyranid Carnifex and consists of a flamer like stream of highly energised particles issued from the creatures mouth,
The carnifex generates the fuel for this inside its own body where it is stored and released. It is this release and the accompanying sound, something similar to a high pitched scream caused by the highly energised material being literally vomited out of the create, that has lead to its nickname of the "screamer killer".
Tactics
Using the scream uses up all the carnifex's bio-ammunition in one blast, the more bio-ammunition you have the more powerful the attack is. Screaming as rapidly as possible does not do as much damage as the occasional howler. Using a combination of a fully charged scream and talon slash as Screamer-killer can kill most marine classes ina a single hit, you just have to get close enough.