They appeared as gray skinned, red eyed Autobots and Decepticons who were mindless zombies that did the Dweller's bidding by attacking other Transformers and draining their energy through a simple touch. Those who suffered from suck attacks became energy vampires themselves and they would return to the Dweller where they gave the captured energy to the ever hungry monster.
According to Perceptor, those who become energy vampires suffer from a unique form of energy depletion which makes them energy leeches. A massive powerload was capable of bringing them back to normal.
The Transformers cartoon
Once the Dweller was released from its prison, it began draining the energy of any Transformer it attacked. Those who suffered from the attacks became energy vampires which began to attack others on Cybertron.
Fun Productions Cybertron Comic
Scorponok awakens on Cybertron with a thirst for Energon and living Sparks. Ramjet comments that he has seen Spark maladies of this sort before, and that Scorponok's victims will become energy vampires themselves.