No, this happened a while back. Riombara captured Overlord in battle, but instead of Overlord appearing in our Dungeons it was one of his Sons. He basically swapped places to avoid imprisonment. He has done the same in Pomatim. Basically, the killing blow was dealt to the 10th Son, as Overlord swapped places with him (Or at least, changed the 10th's Son location with his). Technically, the Human Realms did not kill Overlord in Pomatim, they killed the 10th Son. Of course, for now, this was the last of his Sons, at least on Beluaterra. His safety measure is gone at least temporarily.
I assume the Overlord will reappear soon enough, alive and kicking, ready to continue his conquest. Then the Human Rulers will gloat about killing him, and the Overlord will comment how insignificant that is.