I'm a player through this campaign, and our group is nearing the end of our quest -- we just defeated the clay golem and undead guardian on the lowest level, and we've found the array that holds the lenses. Which leads me to to this post.
Our GM has stated that because the lenses have the "Possession" trait, they give us access to the possession spell. In other words, once we activate them, it will allow us to "project [our] mind and spirit into a target" (according to the trait description and very similar spell description).
He further explained that the array would work similar to the possession spell: if we hit a target -- ANY target, not just Belcorra -- with the ray from the combined lenses, the body of whoever holds the array would become limp while their spirit will be in control of the target for up to a minute.
Of course, GM word is final, so that is how it will play out for our final stretch of this adventure. It just seems to me this interpretation is rife for abuse; it will allow us to take out almost anyone in our path, at the cost of losing a party member for the duration of a fight.
This post, then, is for my own edification more than anything. Is our GM correct? Or is the Possession trait simply a tag to express how the essences of Nhimbaloth will affect Belcorra in the final fight?
Your Gm has done their own spin on the lenses. The lenses give certain abilities when invested. And you can forcefully invest them into someone else, with a successful melee attack (with a set attack modifier depending on the lens, unless you have them in the lattice which then gives all the highest modifier). The lenses don't have the possession trait, however, the attack to forcefully invest them into someone else does. Any they don't give access to the possession spell, as written.
Possession is a level 7 spell which is quite powerful to have free access to for a level 9-10 party (seeing as it is available at level 13 for a caster). Maybe your GM thought the lenses needed a boost to make them more attractive, or that your party needed one. The possession spell does have the incapacitation trait, but since it's a level 7 spell that won't really dampen its power much in this adventure.