Questions About Omega / Nero in Dirge of Cerberus

I was a little confused about Omega as well as Nero in Dirge of Cerberus. When we see it at some point, there are shadow tentacles. Was this due to the merge between Nero and Weiss? Also, what was the deal with Nero’s power? Was he actually able to open a portal to another dimension (seemingly a dark, empty one) as well as travel and bring captives to other dimensions as seen with the battle with him in that area with the rocks, lava and chains? And I guess my final question would be does Omega have that power after the merger?