When the Swampies were first evicted from Delta Magna and given the planet's third moon as a sort of reservation, they took with them a holy relic that enabled the one who held it to see the future, and also a couple specimens of the giant squid that they worshipped. One of these squids swallowed the relic along with the high priest and subsequently grew to over two square miles in size, with a central mass that measured a quarter mile in diameter and 140 feet high. The Swampies called this creature Kroll.
Kroll usually sleeps at the bottom of a large lake, and it stays dormant for centuries at a time. Its feeding processes continue while it sleeps, through its tentacles. When awake, it can travel under the swamp as easily as through water, and it has a highly sensitive mechanism for detecting food. Kroll hunts by surface vibrations and can be confused by excessive noise. It is primarily vegetarian, but eventually started eating anything that moved. It makes a high-pitched sound, a cross between squeaking and creaking, has a mouth full of large teeth, and has at least thirty tentacles on one side alone. It seems to have only a few very large tentacles, with many other smaller ones which it uses to snare its food.
The Swampies worshipped Kroll for centuries, calling it the "Great One" and considering it their protector. Besides praying to Kroll, they made blood sacrifices to it before battle. This involved a ritual in which the Swampies danced and shouted Kroll's name while the sacrifice, bound to the stone of blood, was confronted by a Swampy in a Kroll costume and throttled. The Swampies rationalized this bit of deception by claiming that Kroll's servants appeared in his guise to do as he bid them. As the Swampies seemed to worship Kroll alone, it seems likely that Kroll had also consumed the other giant squids of Delta Magna III. The Swampies considered the mark of Kroll to be sacred, and it was used on the staff of their High Priest. Kroll apparently rose out of the lake three times in Swampy history, and the third instance was recorded in the Swampies' holy Old Book:
"When Kroll awakened, he saw that the people were fat and indolent. And then Kroll became angry, and he struck them down, swallowing into him the symbol of his power, and killing all who were in the temple, even Harges the priest. Great was the lamentation of the people, but Kroll returned to the water and slept. Thus was the third manifestation of Kroll."
When the Doctor and Romana arrived on Delta Magna III in time for Kroll's fourth manifestation, they were slow to believe the legend, although the indentations of ancient sucker marks in stone did lend credibility to it. In fact, Kroll had already been awakened by a methane refinery on the edge of the lake. The staff of the refinery were able to process 100 tons of compressed protein twice a day from the amount of methane Kroll excreted, and estimated that the current levels could support at least ten more refineries. But the refinery's heat exchanges had raised the temperature of the lake by several degrees, and this, combined with the noise from orbit shots delivering the protein, awakened Kroll. Several men from the refinery drilled into the sediment to take methane samples, and Kroll ate them. It began to stalk the surrounding countryside, tearing apart the refinery and the Swampy village in its quest for food. When the Doctor realized that the symbol of Kroll's power was actually the fifth segment, he removed it from Kroll's body. Through cellular regeneration, Kroll's huge mass was converted into hundreds of baby giant squids, which would eventually grow to repopulate the moon.
