X-Men: What Happened To Everyone Who Became The Phoenix?

After he had stolen a portion of the Phoenix Force thanks in part to Feron, Necrom used the shard to create the Anti-Phoenix. After it had matured, Necrom absorbed it and headed to Earth-616.

Here he tried to grab the whole enchilada for himself and attacked Rachel Summers but, unfortunately for him, Rachel and the Phoenix Force were too strong for him and he was disintegrated - his atoms scattered across space in the process.