Melkur, literally, "a fly trapped in honey". The peaceful atmosphere of Traken rendered them immobile and harmless. There were several Melkurs attracted to Traken, most decayed without causing any long term harm. They were rendered as stone through the imobilisation.
The Melkur that seduced Kassia was more virulent than previous Melkurs and was revealed, shortly after the arrival of the Doctor and Adric, to be the Master's TARDIS. The Master utilised the Melkur form to become Keeper and take control of The Source. Nyssa and Tegan saw an illusion of the Melkur intended to frighten them.