Ancient Egypt: Selina follows the Winx to Egypt, a never-ending desertic landscape with Pyramids and carved pillars. This makes very predictable the monster she summons: The Sphinx.
Mythology Gag: Bloom's outfit in the flashback is her season 1 outfit. In the Nick continuity, It's the first time it's ever shown up.
The Reveal: Selina and Bloom are revealed to be childhood friends.