This was a fantastic immersion into ancient Egypt, made utterly believable by your incredible storytelling ability. As others have said, the time you spent in research must have been significant, but we're certainly not beaten over the head with it.
This entire world felt natural, as though I'd been dropped into it and made to stand next to the characters. Our first introduction to Severus is actually a scene which stood out very sharply for me, both the descriptions of the place and the way the confrontation played out. I liked that Harry went with his gut instinct (as he so often does in canon) and both characters felt true to form. Severus as the slave, knowing his place and yet still proud even as he knelt, injured, at Harry's feet. Just..... *insert unintelligible keyboard key crash here*
Harry, allowing himself to be punished for his slave *another keyboard crash*
What I liked best about this story is that we can still identify pieces of canon within the tale, which to me shows a great ability as a writer to take two worlds so different and yet make us,the reader, so comfortable and... dare I say 'familiar' with everything.