Slayn was the original homeworld of the Tal'darim.
Following the protoss rebellion, the Xel'Naga left Aiur with what little numbers they had left. Despite their own creations rebelling against them, a group of protoss willingly went with the Xel'Naga. These protoss ultimately settled on Slayn, and became known as the Tal'darim.
The planet's surface originally had vast jungles, but these were eventually damaged and destroyed by much exposure to terrazine, which began flooding the atmosphere through cracks in the surface. The Tal'darim used the terrazine to communicate with Amon.
When Artanis arrived to Slayn, he helped Alarak challenge Malash in the ritual known as Rak'Shir. All Artanis had to do was keep Malash's forces away from Alarak while he attempted to push Malash into the pit. The Tal'darim would then leave Slayn and follow the Daelaam to Aiur.
After Amon's defeat, Alarak declined an alliance with the Daelaam. He and the Tal'darim set out to find a new homeworld. Despite this, Alarak allowed his people a chance to join the Daelaam.