The Watchers are a mysterious group of otherworldly beings. They are led by the Queen-beast, also known as the Grotesquerie Queen. They were created as the servants and agents of the Gods, being their instruments in destroying humanity. The meaning of the Cult of the Watchers' commandments is not that it would be sacrilege to depict the Watchers, but rather that even doing nothing, belief (or rather, infection) will naturally spread.
During the age of the Intoners, an Intoner and her disciple could combine their powers to summon them from the great beyond. Each Intoner and disciple had a different appearance, ability and summoning pact for their respective angel. A disciple could choose to summon them without the support of their Intoner at the cost of losing their human sentience.
They are an object of worship for the Empire, who are governed by the Cult of the Watchers. The Watchers display various divine abilities throughout the course of the series. It is clear they are able to possess the weak minded. This possession can be long-lasting or short term, depending on the host.