Charger Grub

Charger Grubs are alien larva-like creatures that feed on lightning energy. They are native to the Elemental Plane of Air, where they live as parasites for electric beings such as elementals and thunderbirds. The grubs are insatiable in their hunger, often draining out the creature they attach to.
Their skin is milky blue and translucent, and they swell as they gather energy. If harmed, they release the electricity through their skin and shock their aggressor.
According to legends, only rarely does a charger grub gather enough energy to pupate. From this pupa emerges a terrifying giant insectoid known as the Thunder Beetle.
Lately charger grubs have been sighted gnawing on power cords in Magnapolis, but nobody seems to know where they came from.