So, Xarxes is a god of the Aldmeri, either a scribe to Auriel or (as per http://www.imperial-library.info/content/tuwhacca-arkay-xarxes) a priest that ascended to godhood. He wrote the Oghma Infinium with the knowledge granted to him by Hermaeus Mora (and/or, created his wife Oghma out of his favorite moments of history). Among the secrets Hermaeus Mora has hidden away include information on CHIM (the black book Waking Dreams in particular)

I'm unsure of Xarxes's relationship with Mehrunes Dagon's Mysterium Xarxes aside from the name, but Mankar Camoran's http://www.imperial-library.info/content/mythic-dawn-commentaries directly mention CHIM.

ESO connects the Merish deity Xarxes with the Mannish deity http://www.imperial-library.info/content/arkay-god: a mortal man with a lust for knowledge, granted godhood by Mara so that he could have more time to learn the secrets of life and death, and act as the god for life and death. When Mara asked him what the reason for death and birth was, this was Arkay's answer:

This actually resonates really well with one of Lorkhan's purported motives for creating the mortal plane: creating the potential for beings to ascend beyond godhood (CHIM, Amaranth), which wasn't possible before.

Now, there's the question of if Arkay ever was a mortal to begin with. The Monomyth states that Arkay was one of the first spirits to crystallize after Akatosh created linear time: the question is, why would a god of birth and death (The Mortals' God, no less) come into being before mortality was even a concept? Mannimarco's manifesto, http://www.imperial-library.info/content/arkay-enemy-0, describes Arkay thusly: "Once he was also just a man. The similarities between his mortal life and my own astonish even me. It is only proper that we should be enemies." Mannimarco's instructions to his followers have similarities to the Thalmor's agenda on Talos worship: "To the Scholars: Humiliate the priests of Arkay. Reveal their primitive burial customs to be mere superstition. Befriend kings with honeyed words and bind them to your will. Look to my children in Cyrodiil for guidance."

It could be possible that, before the creation of Mundus, Arkay was a god of cycles (seasons, and then birth and death, among others) and simply adopted the cycle of birth and death when that concept came into being. Or, the mortal who became Arkay replaced/combined with/mantled Arkay the god and laid out how the cycle of birth and death should work.

My question is, did Xarxes or Arkay achieve CHIM? It seems like Xarxes would definitely have knowledge of CHIM, and their ascension to godhood and the rules Arkay sets as a god seem to support that. Talos and Vivec have similar stories, although I think Mannimarco only ascended to godhood, and not CHIM.