As an action, you can ask for the intervention of one of your Watchers.
To persuade a Watcher to help, roll an animal handling check. The DC will be set by the difficulty of what is being asked if the Watcher. Watchers can pass through walls but cannot directly interact with the material plane in any way.
The watcher has the same statistics of a coiled serpent, though it is a fiend.
In combat, a charmed rolls its own initiative and acts on its own turn. A Watcher can't attack or directly interact with the material plane, but it can take other actions as normal.
When the Watcher drops to 0 hit points, it disappears, leaving behind no physical form.
You can only interact with a Watcher a number of times equal to your proficiency bonus per long rest. Upon a failed animal handling check, the Watchers will ignore you until you finish a long rest.