The Arathim were a spider-like species who lived in Thra during the Age of Division.
The Arathim initially lived in the Caves of Grot, before being banished to the catacombs of the Castle of the Crystal by the Skeksis when they refused to serve them. The Skeksis then handed Grot to the ancestors of the Grottan Clan, thus causing the Arathim to feel bitter towards Gelfling.
Many orbital periods later, they've fought against the Stonewood Clan in the Battle of Stone-in-the-Wood, where they were defeated.
Thirty orbital periods later, SkekSo the Emperor convinced the Arathim to join the Skeksis in their war against the Gelfling Resistance, promising to return their ancient homeland to them. Arathim forces lead by SkekVar successfully defeated the Stonewood Rebellion, and then headed to Grot, unaware that the caves had been infected by the Darkening and that the Skeksis intended for them to die there. Fortunately, the Skeksis' plan was discovered by Rian and Deet, who convinced the Arathim to join the Resistance.
After the battle, SkekTek created the first Garthim using Gruenak and Arathim corpses.