Book: Special Weapons.
Fireteam Tuyet died in the Ishtar Sink, hunting the secrets of the Vex. They must have come too close to something precious, for the Vex descended on them with their typical inscrutable, thorough violence.
But their sacrifice was not in vain. The data they gathered helped forge the Pocket Infinity. Properly modified, the weapon should be capable of devastating output on just a single charge cycle.
The Infinity's mechanisms have proven difficult, if not impossible, to replicate en masse. It is conceivable that the weapon draws its energy from the Vex networks...an ominous possibility. So be wary with it.