@rechonq The realms can't normally be seen, no, their existence would be totally hidden if not for the power of deities like Amgarrack. Amgarrack is the only God of over a trillion beings spread across the stars. Yet he's a passive God, he deals in blessings and fortunes, his offensive abilities are unimaginative and weak, his primary abilities being to 'buff' others.
Our enemy is old and has a different source of power besides followers, he's also evil and aggressive, so he is of similar caliber to Amgarrack, but stronger and due to his spheres more clever and well equipped for the trickery needed to set a disaster of this scale in motion.
As for hiding realms, it may be worth moving them to Earth for a time. That could certainly work as hiding it, but then you have the problem of being on Earth... Maybe if a god had a sphere around hiding things, or just magic in general. Alternatively the realm could simply be altered to be more defensible. Occus transformed Hyperborea into the magic forest it is now using a combination of magic and a piece of his soul. The piece of soul, once disconnected from Occus when he died, forced the realm to look after itself and it became sentient enough to take steps to protect itself.
It's a unique case because magic was involved, and unguided magic does crazy things, but the process could be replicated in order to essentially 'edit' a realm. Tapping into the essence of the realm and binding it to yourself would allow some control over it. Influencing the terrain, the elements, etc. I do suggest being ready to sever he bond once the changes are made, or if you plan to bring the ability to shape the battlefield with you into battle, be ready to sever it at a moments notice. You don't want to be connected to a realm about to be ripped apart.
I really need to make a chart, but I'm just going to stick this into the Divine Magic category of my mental list. The kind of magic available to all the gods regardless of spheres. Things like creation, inspiration, worship buffs, a superior form, shapeshifting (to a degree), and immortality (in the sense that your soul will never be fully destroyed).
This would be something that had never been done in the presence of the Merged before, something not done since long before they even existed. So the idea would have to come to whoever used this somehow. It's not just something everyone knows they can do or even how to do it. A forgotten art you could say.