(Uatu) Fantastic Four #13 (1963); (as species) Strange Tales #53 (1964)
Their home-world exists in an unknown galaxy, far beyond the Milky Way Galaxy. It is covered with lush vegetation and bears no signs of intelligence, life, or habitation.
The Watchers possess advanced technology existing billions of years before most other species. They used their advanced technology to create temporary dwellings which they disassemble after use. They also possess technology to observe alternate realities.
Ruled under leader The One.
The Watchers are humanoid beings who dress in clothing resembling that of ancient monks or priests.