Echoes of the Eye introduces a second alien civilisation — the Owlks — who discovered the Eye of the Universe before the Nomai and, in response, created a sealed simulation to hide within rather than pursue the Eye’s invitation to end the universe. Their choice was preservation over discovery.
The final encounter — the Prisoner — is an Owlk who was imprisoned by their own civilisation for trying to warn the Nomai about the Eye and the Simulation. The Prisoner made contact with the Nomai, hoping to create collaboration; their civilisation considered this a betrayal of the simulation’s concealment and imprisoned them.
The encounter has no combat. It requires putting down all light sources — the Owlks are terrified of fire-based light — and approaching in darkness. The Prisoner tells their story through shared memory, and the resolution is the player sitting with the Prisoner until the memories are complete.
Mobius Digital built the expansion’s climax around sitting quietly in the dark with a being who has been imprisoned for trying to do the right thing. The Prisoner’s apology — offered to the player as representative of the cosmic connection the Prisoner sought — is delivered in silence and received without words.

Detail work in areas most players never visit separates great games from good ones.
200 hours in and I never caught this. The developers really reward obsessive players.