So this just popped into my head.

Theory:

We know that the seven suns in the real Tomb ​of Ariel are gone, likely stolen by the Vile Thieving Bird (VTB). Why? Beyond its obsession with shiny things, it might be directly related to its spawn.

In the Third Nightmare, the VTB didn't just steal Sunny’s \[Fated\] it also made off with the \[Shadow of the VTB Spawn\]. This leads to the theory that after escaping into the real world, the VTB needed a way to hatch its egg.

If we look back to the chapter where Sunny first encountered the spawn's egg, it tried to consume his essence. However, because Sunny possesses Shadow Essence rather than standard soul essence, the egg "died". It was only able to be alive because of the soul devouring tree.

If the same rules apply to the VTB’s current spawn, it would require a massive amount of pure soul essence to hatch. The seven divine soul cores (the suns) in the Tomb of Ariel provide the perfect power source an incredibly concentrated and pure supply of essence to hatch the egg.

Resulting in the being no more suns in the ToA.

Why hasn't it left the ToA?

It is guarding its nest. Every bird needs a nest, usually made of straw or grass, to protect its young. For a creature that loves shiny objects and legendary treasures, what better material is there than the golden strings of Sunny’s \[Fated\].

It is also possible the VTB is using the Nightmare Spell itself to facilitate the hatching process. 

Also could be why the river it not flowing because it source power, the 7 suns are in the estuary where everything is static.

Honestly, this could be completely true, it makes perfect sense.



I HAVE FIGURED IT OUT!!!!! VTB and it's Spawn, Real ToA and Spell.