Insert spoiler song and dance.
First of all, I was glad to see the Doctor and Clara patch up their relationship. Clara’s outburst at the end of “Kill the Moon” was stunning to say the least, and I wondered if she would even be in this episode. Still, I doubt the Doctor will take it well when he learns Clara lied to him about Danny.
As for the mummy itself, I was sort of expecting a “Pyramids of Mars”-style mummy, but this was something different. It actually reminded me of a line from “The End of Time” where one of the Time Lords says that the soldiers keep dying, “with time itself finding new ways of resurrecting them.” In fact, I would almost wonder if this was one of the soldiers from the Time War if it weren’t for the fact that the Doctor didn’t recognize the flag or the technology keeping the mummy alive.
And creepy though it was, I think my favorite effect was the scene where the mummy is “passing through” the Doctor. That just looked cool.