Eberron: Rising from the Last War includes a really interesting boss monster named Belashyrra.
Belashyrra has Truesight out to 120 feet:
A monster with truesight can, out to a specific range, see in normal and magical darkness, see invisible creatures and objects, automatically detect visual illusions and succeed on saving throws against them, and perceive the original form of a shapechanger or a creature that is transformed by magic. Furthermore, the monster can see into the Ethereal Plane within the same range.
Belahsyrra also has a powerful ability called Eye Thief:
Belashyrra can see through the eyes of all creatures within 120 feet of it. It can use its Eye Ray through any creature within 120 feet of it, as though it were in that creature’s space.
The interaction of these two abilities, Truesight and Eye Thief, raises a couple questions:
Does Belashyrra still have Truesight when viewing through another creature's eyes? This question is important for adjudicating an invisible creature behind total cover. Truesight let's you see invisible creatures, but if it doesn't apply when using Eye Thief, Belashyrra wouldn't be able to see an invisible creature behind total cover, even when looking through their eyes.
If Truesight does work when viewing through another creature's eyes, is the 120 foot range measured still from Belashyrra's position, or the creature's?