I don't see anything behind the Selenavatar, so presumably that blade is emerging from within. Based solely on the blade, I'd guess some sort of Nyd weapon. Possibly Daemonic, as it sort of resembles the Daemonette Eldar child we saw a few pages back.
The pattern of the blade is similar to Selene's scythe, but the texture is markedly different. It's also much larger than it should be. If it is "Selene" emerging, she's gotten a lot bigger, and probably undergone a number of other changes.