Isn't Mithra a God of Friendship? TF does it gotta do with the End of Days?
everything I'm finding about Mithra relates to Iranian/Persian mythos, wherein Mithra is described as a god of contracts, binding, and is all-seeing/omniscient for similar reasons (and rewards/punishes those who honor/break their oaths and bonds and word and the trust of others). "Friendship" could be a facet of that, and there was some stuff wherein an epithet of Mithra relates to that (and to cattle, incidentally) - but in the context of the character in this series....
There is one tidbit I found about Iranian worship surrounding the Winter Solstice, which is also the birthdate of Mithra--heralding the return of the Sun and the lengthening of days after the longest night of the year. Mithra has connection to the Sun and Light, though there are other deities more directly correlated with the Sun itself. But because of the Solstice connection, Mithra is also seen as a deity of renewal.
If Mithra in this series is seeking to bring about the destruction of the world through Aru and leads a cult surrounding that premise, there could be some language about "resetting the world through annihilation" or something. Doomsday/Death Cults deal in that sort of stuff in real life, as well--the idea of being reborn through death and mass destruction and whatnot.
If there's any real-world connection/inspiration for this character's name and it's related to that specific deity, then that would be guess as to the author's intent (assuming I have the right deity in the first place, though I saw no others popping up in my initial search).