Talk:Shadow Play - Part 2/@comment-1031724-20180515201251/@comment-29981995-20190523224326

@Grimlord what exactly have we learned about Tartarus that makes that seem unlikely? Have you forgotten that Tirek was able to be pen pals with a megalomaniacal Pegasus filly? Also, from Twilight's Kingdom - Part 2:

Princess Celestia: We do not believe that Tirek is aware that a fourth Alicorn princess exists in Equestria. If we transfer our magic to you, Tirek will not know where it has gone.

Between that and the fact that Tirek registered no surprise to see Cadance with Celestia and Luna, I would guess he knew about her.