Kroll was descended from one of the giant squid brought to the moon by the humans. At one point, it ate one of the priests of the Swampies and their holy relic, which was actually part of the Key to Time. This segment mutated Kroll, making it grow to a mile across and a hundred forty feet high, with over thirty tentacles on each side of its body. Though it was mainly a herbivore, it also ate any animals it could sense by vibrations. Kroll spent most of its time under the mud of the moon, passively feeding on plants and animals. It surfaced at least three times, becoming the god of the Swampies. The Swampies called these scattered appearances "manifestations".
The fourth time it surfaced, the moon was inhabited by humans searching for methane. Kroll produced large amounts of methane during his periods of dormancy. This was harvested by the humans. Kroll was woken by the drilling and the firing of the orbit shot.
At this point, the Swampies were trying to take back the moon from the humans and took Kroll's appearances for a sign, even though Kroll ate humans and Swampies indiscriminately. The Swampies grew bolder and attacked the methane plant, killing Thawn. Unfortunately, this commotion attracted Kroll and it attacked the refinery. The Fourth Doctor converted Kroll back into a segment of the Key to Time, leaving a few squid as remains. (TV: The Power of Kroll)