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.