Okay, title might sound a bit crazy, but hear me out.

When you cross the bridge to the Divine Tower of East Altus, you're pulled into a boss fight with two Omens: https://www.youtube.com/watch?v=GVuAtY6Lq8M

!

I think that this fight happens inside the veil which separates the Shadow Lands from the Lands Between. There are a bunch of reasons for this:

1. We know that Omen blood is needed to cross into the Shadow Lands, as Miquella uses Mogh's accursed blood to allow himself and his followers to cross the boundary
2. There are very few examples of similar visual effects - I can only really think of the fight against Rennala (the transition to the second phase), and the fight against the Elden Beast (also transition to second phase). Nowhere else do we see shadows appearing like this, and both these cases can be linked to Marika (through Ranni and Radagon respectively)
3. The Omen don't have the kind of magic you'd need to do something like this. They have some magical properties, but those are mostly out of their control. Maybe they aren't even doing this on purpose - the Fell Twins could simply be stuck inside the veil, unable to progress further
4. When overlaying the DLC and basegame maps, we can be sure the Divine Towers would be close to the borders. Even though the maps don't fit when looking at all details, we can be certain the veil must run somewhere along the area of the divine bridges
5. The fight ends once we kill both Omen. It's reasonable to assume that the hand which transports us to the Shadow Lands also would cease working if all the Omen blood were removed

All of this leads me to believe that this fight happens inside the veil! What do you think?