From the introduction:
The Khùl-Algrenàthi'i or "They Who Watch the Many Planes", are extremely old. Their presence in Ushù'l – and, it appears, most of the rest of Telostic – dates back far beyond the start of recorded history, as it is commonly accepted by modern scholars.
The Watchers are a group of nearly immortal, semi-divine sapient beings, most (but apparently not all) possessing an Aablùshyé or "Pendant of Ruling in Glory Over Reality", supposedly, some sort of extremely powerful magical or technological artifact.
User summary:
By far the shortest volume of the main rules series, The Watchers probably is also the least significant in terms of game play. It also was the last-developed segment of the game and did not appear in any form until the Version 4.1 release.