Lathander is one of the few ancients who will remain even when nothing else does. He has not been seen for a very long time however and for unknown reasons his power has vained.
Noone knows when the multiverse first came to exist nor when Lathander did, all we know is that Lathander once played a major role in the massive events of our early existence. He created, gave, took and planned. Some believe all dieties powers stem from Lathander and that the more dieties that come to exist, the weaker he will become, others believe that he is merely resting from an event so impactful his godpower will be gone for thousands of centuries. None can know.