Black Hole tech and using it.

I read somewhere ( I think codex) that one of the Kabals had tech to create black holes. What's stopping them to pop at Cadia and drop a black hole there or even at Terra?? So what if it stirs the Imperials but a whole chunk of the population including their emperor would be destroyed.

The codex features Vect destroying another Archon's subrealm with a caged singularity, and singularity weapons are mentioned in Path of the Renegade (specifically that you'd have to be an idiot to fall for a such a trap since the amount of energy needed to contain the singularity would be easily detected). In theory I don't see anything stopping Dark Eldar from using singularity weapons (tho I expect such things to be very rare, possibly pre-Fall relics), tho how powerful such weapons are isn't known. We know know one destroyed a subrealm, but we don't know how big the subrealm was and how stable the singularity would be (if it only lasted for a short time it would destroy everything around it before destabilizing, but if it was stable it could destoy the planet it was dropped on).
In any case, there's very little reason for Dark Eldar to drop such a thing on Terra. Doing so would pretty much destroy the Imperium, and with the Imperium torn apart, the various nasty things they've been holding back (mostly Chaos and Tyranids) would have free reign. Getting all their potential slaves corrupted or eaten would be bad for the Dark Eldar's business. Remember that they aren't truly at war with realspace, they're hunting them for fun and profit.

I agree. That kind of tech doesnt meet the needs of what the DE want from realspace. They need the imperium in all its corrupt ineptitude to maintain order over the human worlds and act as meat shields against the likes of the Nids and Necons. Such a weapon would outright kill too many potential victims that could be better used feeding on them. I think the only time such a weapon would be useful would be at an absolute last resort against the likes of the Nids or maybe a Necron capital world (their name escapes me atm) to try topple a Dynasty.

I concur - the DE have the power to destroy planets and cage stars - they don't bother though because that's not what they need or want right now.

Also on the note of how powerful they are: We dont know how big or small the subrealm is, if the Dark City is near infinite it stands to reason that though it is the biggest of them, the others though smaller will still be massive. Large enough that when its destroyed with a single attack it will show their ability is unrivaled by most races.