Today's arcane deity is…OSIRIS!
Osiris was a big-deal god in the Egyptian pantheon back in the days of the Pharaohs well through the time Christianity swept that country. He was the great-grandson of Ra, who's, like, THE GOD for Egypt during this time. Osiris was the first Pharaoh of Egypt. And all Pharaohs were descended from Osiris, hence they were descended from Ra. Get it?
Osiris is frequently depicted as having green skin and wearing an atef crown w/ ram horns on it. What's an atef crown? Well, remember those pictures of Pharaohs with the coney things on their heads? That, my friend, is an atef crown. (If my stunning descriptive ability isn't doing it for ya, fine).
So. Osiris was also the god of the underworld. How does one go from being a Pharaoh to being god of the dead? Well, yeah. Not a happy story. See, Osiris had a brother, Set, who didn't like him very much. (wow, siblings who mess with each other. Familiar?) Set got Osiris drunk one night and then locked him in a magic box. Then Set poured melted lead into the box, killing Osiris. Set disposed of the box by tossing it into the Nile. Mafiosos would later streamline and economize this idea with their innovative cement shoes method.
Set then forcibly took over the rulership of Egypt. That included forcing Osiris' grieving wife Isis to marry him. Isis, however, would not give up. She devised a super-complicated set of magic moves to bring Osiris back to life. Seriously. In summary, hijinks ensued. In longer summary, the hijinks included getting pregnant by the spirit of her dead husband ( ring a bell?) , getting her sister and no less than two other gods involved, being found out by Set, enduring Set ripping Osiris' body up into a bunch of pieces, sewing the pieces back together, and then finally coaxing Osiris' spirit back into his body.
HOWEVER! Osiris still couldn't come back to the land of the living, because he'd been dead. So he became the lord of the underworld. And the baby that was born from the crazy spirit-pregnancy? A boy, named Horus. Horus eventually defeated Set and took the throne from him. And in the really lovely words of one awesomely stitched-together version of the myth of Osiris, "so it is that Horus watches over us while we live, and gives guidance to the Pharaoh while he lives, and his father Osiris watches over us in the next life. So it is that the gods are at peace". And so it is that many years later, Willow will use the urn of Osiris to bring Buffy back from the dead.
Bonus side-effect from the whole Isis bringing Osiris back to life thing: a promise that all believers can live forever upon Osiris' triumphant return, once Horus finally permanently defeats crazy-ass uncle Set, thus forever vanquishing evil. (again, I ask: Ring a bell?)
And that's...today in Arcane Deities!