Probably the most interesting new character. We've been given just a hint of info about him:
His name, Marethyu, and it's meaning of "death" in the Language of Danu Tallis.
He was on the Island, and Tammuz doesn't like him - but also seemed to survive the fall, as he was around to teach Alchemy to Flamel (and tell him his destiny laid in the prophecy), to help Germain learn Fire Mastery (which was to help him survive for later), has told Perenelle her fortune and that of the world (see Warlock preview) and sworn her to secrecy about it, as well as tell Dee HIS fortune, and that it laid in the Swords.
We know he's orchestrating big things - the massive effort to gather the Warriors to fight with the Original Twins on Danu Talis to cause the fall - a sentance that alone shatters the world, as we'd been told that the Island fell from the battle between Sun and Moon (Dagon even was there to see it...).
The fact that they need warriors they can trust, and seem to be working together to ensure the fall is huge.
But beyond that whole plot - which involved the creation of Shadowrealms, centuries of planning, and carefully sequenced time travel - that doesn't even seem to be the limit of his machinations.
We still don't know if he's Elder, Immortal, or something else entirely (although Sophie through the Witch knows both his name "and WHAT he is..." [The Magician].)
And Josh's flashback of him and a red haired warrior fighitng Coatlcue is also significant, but not sure how.