Valender, the godfather of Valendri, a half-elf half-human who united multiple elf societies to rival Nethero. Valender collected forgotten lore and sought knowledge from the outer planes. He tried to create a plan of pure order and harmony. In doing so contacted Mechanus. Certain offspring are blessed with some of the same power. He loved Sundaria and after having a daughter, Solara. The Phaerimm took Sundaria as a host and he eventually left Valendri to stop the Phaerimm. It was then that he took up the name Karsus.
In the Age of Arcanum, Karsus crafted a ritual, known as Karsus’ Avatar, to allow him to assume the mantle of a god. He did this during the The Spellplague while Phaerimm were wreaking havoc. He killed thousands in the process of creating this avatar, and chose to become the god of magic, Mystra. In doing so broke her concentration on the weave. This destroyed all magic and Mystra herself. In that moment a new god/goddess of magic was born (and placed the limitation we now know on magic) and Karsus died, falling into the sea. The floating cities, all but Valendri chosen by Karsus himself to be safe, fell as well, killing thousands. His body makes up the land, but is suspected to not be inert. It is believed that the heart of Karsus could be used to create another avatar. Many wizards believed that his ritual could have been successful if he chosen another god’s mantle to assume, but it is now known that he intentionally chose Mystra to starve the Phaerimm of the weave.
He would not have subscribed to the way Valendri is now, did not believe they should be an isolationist nation and wanted to unite people. Was close friends with Solonor who states that his only true folly was saving the Phaerimm that consumed Valender’s love Sundaria.