Possibility 1: Jagex doesn't bring him back.
They spent all of the Elder God Wars story line either killing off or exiling gods or nerfing characters (along with elder artifacts).
Zaros going to Erebus may have just been a way to shuffle him off and conveniently never have to bring him back. (He spent thousands of years from the time Zamorak stabbed him just to be a powerful orb of T2 energy again. Going to Erebus and whatever his plan is might not happen within the player character's lifetime).
Possibility 2: Zaros was Xau-Tak all along
He had at least 1 Elder Artifact with him. Unkown/untold knowledge. Mah's core. Raksha's thing. Now Jas. Plus everything in Erebus. We also know that they said that a trail of the shadow realm extended from Freneskae to Gielinor.
Jagex could always do some fudging of the storyline to indicate that Xau-Tak and Zaros were always one and the same. (Xau-Tak let slip stories of the player character before the PC was even born. Xau-Tak has a hatred of Elder Gods and wants power/control. We have time travel. It's all one convoluted possibility to explain things.)
Possibility 3: He's dead
Much like 1, but rather than the vagueness of never having to bring him back alive ... you have the classic trope of "nothing could have survived that" (keep in mind, the player wasn't in Erebus for that long compared to Zaros' plan).
Jagex could just treat him as "yup, he'd dead. he got to arrogant and despite all his planning, preparation, and even being a shadow element himself, even he couldn't survive in there forever".
From the Wiki
In Sliske's Endgame, Death can be asked how Zaros would die. Death cryptically claims that he feels "a burning as hot as a star, and then a giant blackness. I feel a prolonged sadness that has come to an end."
pretty easy to see the course of events playing out in the way Death predicted
20-Jan-2023 14:37:14
