Hey, I've been playing the Umbral Choir a lot, and I have come up with some UC challenges that aren't achievments but I think are pretty cool.

1. Build a sanctuary on every planet of an enemy home system

Pretty self explanitory.

2. Fire an oblitorator and destroy a system before anyone knows you exist

This one requires some carful planning, as Oblitorators can't be cloaked, so you need to keep them out of sight while they charge.

(FYI as soon as you do this, the AI seems to send scout ships toward the point it was fired from. Bonus points for you if you can stay hidden after the fireworks start.)

3. Abduct every population (down to 0) in an enemy system anonymously

Yes, there are ways to pull this off without revealing yourself. 

4. Eliminate an enemy who has never met you - *You are not allowed to Oblitorate their final system. 

Again, there is a way. You will have to combine existing gameplay mechanics into the UC gameplay to pull off these last 2. Be creative.

One major point to help with these: Alliances are the enemy of the UC. If you are revealed to one enemy, and he joins an alliance, suddenly you are known to 3 enemies. Carful not to reveal yourself to anyone. It's a long, lonely road to the top.