The Dweller is a large Trans-Organic that was created by the Quintessons in the distant past on Cybertron. Due to its violent and dangerous nature, it was sealed by the Quintessons for all of time. Like a vampire, it has the power to drain the life-force from anything it touches. It is virtually indestructible and not very nice at all.
The Transformers cartoon continuity
Long before the Quintessons had even created the robots which would one day become the Transformers, their early experiments centered around a fusion of technology and organic material. One of their creations was an unstoppable abomination known as the Dweller. They managed to defeat the Dweller and the rest of the Techno-Organic monsters and seal them away deep within the bowels of Cybertron. However, it was accidentally awakened as part of a Quintesson trap against the Transformers.
After encountering Galvatron, the Dweller cut a swath of destruction across Cybertron until it attacked the new Autobot power plant. The Autobots used it to dispose of the creature by ejecting the power plant into space where it crashed into a Quintesson ship that was monitoring the events. The fate of this creature is unknown after it crashed into the Quintesson ship.
3H Wreckers comics
The Dinobots were sent to an unnamed planet by the Oracle in order to awaken a "sleeping giant" for a mysterious purpose. However, the mission was a fool's errand, arranged by the Quintessons and designed to destroy the Dinobots. The planet was the world where one of their most dangerous creations ever remained a prisoner, the energy-draining Trans-Organic creature known as the Dweller. Airraptor, Rapticon, and Magmatron were subsequently destroyed in the deadly encounter.