Do we still have the Assassinorum request? Would be perfect for a Night Lord/Alpha Legion chapter. I'd call them the Nightstalkers. Some ideas for planets.
[ ] Terrararaaa: A planet that looks mostly normal on the surface, but in truth hosts a cave network so utterly massive in scale that the Mechanicus still has not figured out how the entire thing maintains consistent gravity. You also haven't figured out how it doesn't collapse on itself from the pressure of gravity. The locals on this planet are semi-civilized, and spend most of their time fighting the uncountable horrors from the depths.(Segmentum Solar)
The caves could be an interesting place for a hidden fortress,, kinda like Bat caves lol.
[ ] Ikkite: A forest planet notable for being covered in spiky, nearly indestructible plant life, the humans of this planet have learned to keep constant awareness of their surroundings even when in the fiercest of combat, lest they be impaled. (Near Terra).
Training perception would make them terrifying, even more than they would already.
[ ] Gloamaz: A planet where the entire surface is covered in dark, gloomy forests. The sun rarely shines on this world, and it's people are a very pale skinned folk. Due to predation by a wide variety of forest monsters, including some kind of 'wolf men' your juniors didn't manage to get a subject of, the people here almost universally live near either a military fort or, more commonly, one of the many castles of the local nobility. (Sanguine).
Not sure about this one. Feel like we should save it for if we get Space Corgi seed, but it does fit the theme.