The Purple Worm is a literal giant purple worm. It was the main reason Refuge was destroyed in every loop.
Pre-Eleventh Hour Edit
Prior to events of The Eleventh Hour arc, Cassidy and a small team of miners headed down into Refuge's quarry in search of diamonds, but instead of diamonds, they found a worm and a nest cut in half by the bubble over Refuge. They ended up waking said worm with their digging, and it attempted to attack them, only to be stopped by an unknown force.
The Eleventh Hour Edit
The Purple Worm was mentioned before when it hit 12:00 pm in Refuge, but the first time it's actually shown up close is in Ep. 44, when the Tres Horny Boys head into the quarry. There is where they get to see the full extent of what they're dealing with.
Later on in the arc, it is revealed that the Purple Worm is a mother, and that it's just trying to get back to its children. Unfortunately, there just happens to be a small town in the way. After a long chase through the mines, the boys manage to pop the bubble, and lead the Purple Worm out of Refuge, where it is reunited with its worm babies. Taking its babies, it then leaves Refuge for good.