Kroll was a gigantic creature that was found on one of the moons of Delta Magna. It 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, causing it to grow to a mile across and 140 feet high, with over 30 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 a major god for the Swampies.
The fourth time it surfaced, the moon was inhabited by Humans searching for methane. At this point, the Swampies were trying to take back the moon from the Humans and took Kroll's appearances for a sign (though Kroll ate both Humans and Swampies indiscriminately). They became more bold and attacked the methane plant, killing Thawn. Unfortunately, this commotion attracted Kroll and it attacked the refinery. The Doctor was able to convert Kroll back into a segment of the Key to Time, leaving a few squid as remains.