Watcher

The Watchers, also known as the Frozen Watchers, are ancient beings who once presided in the Freljord. They were responsible for transforming Freljord native humans in to a new race known as the Iceborn, who would later rebel against The Watcher's rule. The Howling Abyss was built for The Watchers by the Iceborn, in addition to many structures across the Freljord, which now lie in ruins.

Creating the Iceborn
The Watchers origins are unknown, as Lissandra, the only living Iceborn states even she does not know where they came from. The Watchers were immensely powerful, and transformed humans of the Freljord into the immortal Iceborn. The Iceborn were given great gifts, magic and immortality in return for their eternal loyalty. They conquered for The Watchers and built them a great civilisation, including the bridge and fortress looming over the Howling Abyss, which eventually amassed in to an enormous empire. Their mouth piece was the Seeker, Lissandra , who was, and still is fervently loyal to them.

Fall of the Watchers
Avarosa, believing that their servitude to The Watchers was no different to being their slaves, led the Iceborn in rebellion against the Watchers. This culminated in to a long bloody battle across the bridge known as the Howling Abyss. Countless Iceborn were killed, but they eventually won the battle, throwing The Watchers over the side of the bridge, howling in to the abyss, giving the bridge it's name, and paving the way for the Iceborn to rule Freljord.

Erased From History
While many believed The Watchers were dead, Lissandra  did not believe it so. In the midst of the Iceborn civil war known as the War of the Three Sisters, Lissandra killed Avarosa, corrupted a tribe known as the Frostguard, and set about purging any history relating to The Watchers' reign from memory and historical records in preparation for their return. She still actively works to bring them back to the world.

Biology
The Watchers seem to be cyclopean and spouting numerous limbs.

Trivia

 * Cyclopean, statues litter the bridge crossing the Howling Abyss. The monsters carved into the rock look like nothing that the world has seen.
 * was attacked by a creature resembling the Watchers in ancient times.
 * promo, The Wonder from Above, was the first proper depiction of a Watcher shown as a constellation in the night sky.
 * Prior to the reveal of lore and his Q&A, there was a popular theory that Vel'Koz was one of the Frozen Watchers - due to visual similarities and a literal interpretation of Lissandra's quote, "The Watchers were sent howling into The Abyss." It was confirmed that Vel'Koz specifically is not one of Lissandra's Watchers, although a connection between the Watchers and the Void was left up-in-the-air.
 * The Constellation of the Frozen Watchers, in the Northern hemisphere of Runeterra, signalizes the end of the autumn harvest and the beginning of winter.
 * The was created in the image of the Frozen Watchers, and most likely dates back to the time of their reign.
 * Watchers and Iceborn share some similarities with the from the  series. All of them are ice related and all of them signalize the return of winter.