The cutscene showed Bayek talking to Medunamon. Then I played through a short flashback sequence where Bayek was with his son. They hunted together, then went to a cliff where Bayek tried to convince his son to jump to overcome his covardice. Then, one more cutscene where the masked guys captured Bayek and his son and tried to open the crypt. One more cutscene followed where Bayek smashed Medunamon's face with an iron ball. Bam. I have killed Medunamon. Without even knowing how or why this happened.