We don't actually know what it does. I'm just basing my plan of it being a powerful relic of chaos, which means that the outcome will likely be great... and chaotic (if activated!) :D

Lucia had one though, so we know that it has some telepathic powers over whoever makes eye contact with the eye ^^

Plus, being a slaaneshi trinket, expect it to be able to spread lots and lots of sensations! (both euphoric and terrifying!)!