Near the end of the war, Necros was destroyed in a surprise attack by the Renegade Dalek Fleet. Nearly the entire fleet was wiped out in the process, but they were successful in launching an experimental Sinngularity Drive onto the planet's surface, creating an artificial black hole which consumed the planet and all the Imperial Daleks on the world at that time. Any remaining Imperial Daleks later settled on Skaro, only to be wiped out by the Hand of Omega in the conclusion to the war.
Necros's debris field was later inhabited by Humans, and they set up the station of Tranquil Repose, where human bodies were brought to be stored in containment shells rather than be buried. From there, they established one of the many trade routes with other Galaxies, since Necros is located so far out from the Galactic Centre. Ironically, the Daleks themselves would come to attack Tranquil Repose earlier in their timeline, before the Dalek Civil War even happened. That's Time Travel for you!