Some ideas I'd love to do to my players if I still played games like this (I'm old):

Parasitic Immortality: Each player’s body now harbors a sentient, parasitic entity from the elder god's realm. To stay alive, the player must feed the parasite by draining the life of others, and the parasite is very specific of who to consume and when. If they fail, the parasite consumes them from the inside, painfully regenerating them each time but with diminishing humanity.

Memory Reset: The players are indeed immortal, but each time they sleep, they lose a random set of memories or skills. Over centuries, they become strangers to themselves, unsure of their origins, skills, or even motivations. (Think Momento)

Unwanted Guests: Each player's soul is bound to their body forever, but their mind becomes vulnerable to invasion by the elder god's minions. Between play sessions, they find themselves "sharing" their consciousness with malevolent spirits who take control at unpredictable times and start the next play session in terrible or awkward scenarios. (Think GTA V when you switch characters, but more fucked up)

Mosaic Body: Their bodies no longer age, but they start falling apart, requiring constant replacement. They can survive only by patching themselves up with body parts from other beings, creating a grotesque patchwork appearance, and everything hurts. The more messed up ways to gain body parts the better. (Frankenstein's Monster)

Eternal Decay: The god grants them eternal life, but not eternal youth. They’re doomed to experience endless decay, enduring the sensation and accelerated loss of flesh, muscle, and sensation as their bodies remain animated corpses over months instead of decades.

Mirror Life: Between play sessions they wake up in a universe where their actions had the opposite outcome. Friends become enemies, achievements become failures, and they are eternally tormented by a reality where their efforts always yield the worst.

Bound to Territory: They live forever, but their bodies are tethered to a specific region—a cursed ground created by the elder god. If they stray too far, they wither, eventually getting pulled back in by an unseen force to endure perpetual cycles of death and rebirth.

Soul Fracture: Their souls split into multiple pieces, scattered across different planes. They’re immortal, but constantly feel the pain and emotions of all their fractured selves, and each piece they “reclaim” in other worlds is hostile, violent, and deranged.

He Lied: The elder god couldn't be trusted, he did some magic feeling and looking stuff that did nothing just to get these mortals to go away