Horrifying 5 feet long worms that are sickly yellow and covered in viscous, foul-smelling fluid. The head is a distorted face resembling a mortal from a previous life.
Source: Planescape: Monstrous Compendium Appendix I
(Original Source is AD&D Monster Manual)
Larvae are formed when selfish humanoids die and their spirits are doomed to spend their existence in the service of the Lower Planes. Larvae are the building blocks of the Lower Planes. They are treated like herd by the Night Hags who gather them up to be used as a form of currency. Demons and Devils seek out these Larvae to build their fiendish hordes. A Larva sole purpose is to be transformed into a Quasit or Imp.
The Quasits and Imps can eventually transform into other types of fiends. It is not unknown for a greater fiend to transform them into a more powerful demon or devil if needed. The Devils of Baator are known to cast their Larvae into a pool of flame where they suffer for 11 days and emerge as a new cruel creature. Rumors have spread that the Devils have been acquiring millions of Larvae for a foul ritual. The sacrifice of these lowly creatures will fuel a potent spell that will wipe out their hated demonic enemies.
Larvae are semi-intelligent creatures who communicate through body movement that cannot be understood by others.
Combat: A Larva is a low-level opponent worth 35 experience. They have two notable abilities; Wounding and Rotting Disease
-Wounding: Whenever a Larva successfully bites an opponent they continue to bleed for 1 hp of damage each round. The wounds must be bound or magically healed to stop the bleeding.
-Rotting Disease: Anyone bitten must save vs. poison or be infected by a rotting disease. The victim's skin develops a painful skin rot. After 3 weeks the victim loses 4 hit points each day unless they remain absolutely still. After one month they die unless a cure disease is cast upon them.