Anti-matter creature

The anti-matter creature was a being composed of pure anti-matter energy that attacked visitors to Zeta Minor, the planet on the edge of the known universe.

Characteristics
Professor Sorenson described the creature as "pure energy in physical form". The Fourth Doctor remarked that "it [didn't] live anywhere. It just [was]".

Striking only at night, the creature sucked the life out of its victims in a process comparable to rapid freeze-drying. The victim's body would initially disappear before returning as a lifeless, desiccated husk. (TV: Planet of Evil)

History
The creature made a number of attacks on Professor Sorenson's expedition — killing seven of the eight members including Lorenzo, Braun and Baldwin.

Having escaped from Salamar, the Fourth Doctor and Sarah Jane Smith were in the jungle at night and the creature appeared to them. It moved to attack them before electing to kill O'Hara instead.

When the Morestran probe ship tried to leave with anti-matter minerals from the planet on board, it suffered technical failures and the creature attacked, killing several of the crew. It was fended off when, at the Doctor's recommendation, Salamar ordered that the force field be linked to the atomic accelerator.

The Doctor travelled across the jungle to a pool-like structure composed of anti-matter, hoping to negotiate with the creature for the return of the minerals. He was plunged into the pool with the creature and was released due to the anti-matter he had about his person. The Doctor told Sarah he had promised the creature that the mineral would be returned.

However, Sorenson secreted a canister of the minerals aboard the probe ship. He was infected by anti-matter, causing his degeneration into what the Doctor labelled an "anti-man". In order to stop the attacks by Sorenson and the duplicates created when Salamar used a neutron accelerator against it, the minerals and Sorenson submerged in the pool. With the Doctor having kept his promise, the duplicates vanished and Sorenson was returned to his former self. (TV: Planet of Evil)

Other references
Sarah Jane boasted of meeting "anti-matter monsters" to Rose Tyler. (TV: School Reunion)