The Gholos is a gestalt species from Golos.
Gholos usually takes the form of a cloud of gas, and has also been described as looking similar to cotton candy. It often changes colour to suit the occasion; pink normally, red when scared or shocked and blue when attacking. This blue attack, known as "the blue sting of Gholos", can easily kill a human.
Gholos is a gestalt entity, able to divide itself into smaller forms. Spending time away from the gestalt is tiring for it and complete separation is deadly. It uses a highly complex language to communicate. The language of the Gholos is almost impossible to master, and cannot be translated by the TARDIS' telepathic circuits, although at least the Pakhars have developed adequate translators that can do the job.
The Gholos and the Angvian empires were close to each other, leading to territorial disputes. The Angvians claimed their pastoral colonies were attacked by the Gholos, forcing them to go to war. Though peace treaties were made, the Gholos eventually broke them. Likewise, an agreement about the Zordon Nebula was broken by the Angvians. This led to centuries of war.
Eventually, peace was wanted between Golos and Angvia. In order to prevent attacks from Angvian terrorists, two peace conferences were held. One was held on Achilles 4, which was the obvious target. The second was actually the 309th Intergalactic Song Contest at Dark Space 8. The ambassador would have the political information telepathically hidden until they needed to know it.
Despite these measures, the secret got out. Loozly, the translator for the Gholos competitor/ambassador, didn't want peace and tried to disrupt the Song Contest, killing people using Angvian methods. The Gholos eventually learned the truth, but was unable to communicate it and threatened by Loozly with a special device. Eventually, the Seventh Doctor learned the truth and was able to get the device deactivated. When Loozly realised he had lost, he tried to hold the Angvian ambassador hostage. Gholos killed him, saving her and leading towards the peace conference.