The Melkurs were living, but often dormant, statues, animated to be the army of Malador, a being of "pure evil." As such, "Melkur" became the name in Trakenite culture given to corrupted creatures who were attracted to the planet Traken, immediately being frozen in stone upon their arrival.
The Melkurs were created by Malador, their minds extensions of his own, allowing him to see and speak through them, though they still possessed their own intelligence and speech. Given the strength of twenty men, they were indestructible. The Sixth Doctor explained that a Melkur could be thrown into a supernova and come out unscathed. They generated a psychic influence called the "lament of the Melkur," calling out to their kind in search of their master and of evil. The influence was strong enough to pull the Doctor's TARDIS towards them. (AUDIO: The Guardians of Prophecy)
The word Melkur literally meant "a fly caught by honey". The Melkurs attracted to Traken became stone, the peaceful atmosphere of the world rendering them immobile and harmless. Most decayed without causing any long-term harm. (TV: The Keeper of Traken)
Traditionally, a feast took place annually, called the Feast of Melkur, at which those attending participated in poetry, music, dance, recitals and performing arts of all varieties and calibres, to ease its passing. (PROSE: The Astronomer's Apprentice)
The Melkur were created by Malador in the Dark Times as his silent army. After Malador's imprisonment they slept throughout the universe in wait of their master's revival, becoming active when he awoke briefly on the planet Serenity in the Benign Union. (AUDIO: The Guardians of Prophecy)
The Melkur that seduced Kassia was more hostile than previous ones and was revealed, shortly after the arrival of the Fourth Doctor and Adric, to be the Master's TARDIS. The Master utilised the Melkur form to become Keeper and take control of the Source, but was later disconnected. (TV: The Keeper of Traken)
Behind the scenes
The walking Melkur statue seen towards the end of The Keeper of Traken part one was played by Graham Cole, who also played the Melkur illusion seen in Time-Flight part two. On both occasions, he remained uncredited both on-screen and in Radio Times.
Geoffrey Beevers was the voice actor.