"Cousin. Do you use actual lions, celestial or otherwise?"
"Then shut up about us using spears."
- – Aaron Dembski-Bowden's take on the Emperor's Spears
The Emperor's Spears are one of those many, many chapters of whom basically nothing is known other than a name and color scheme. However, it's a totally sweet-ass colour scheme, being pale blue and white with a trident symbol on the pauldron. Because of the obvious nautical themes involved, most fans interpret them as being Poseidon-themed Space Greeks, but in a different way to the Ultramarines.
Aaron Dembski-Bowden is writing a book about them, set after the Gathering Storm. He can't tell us shit, though, because of the NDA; however, the main character is apparently called Vadhán.