In order for an enemy Demigod to go directly to the Cavity, that Demigod would need to be within High Lord Rashal's territory, as he and his people are the only ones who have access. And even then—it would be hard to do, as permission is still needed.
But it's not impossible.
Someone within High Lord Rashal's side could be a mole, working for the other side.
Rex was cautious every step of the way, yes, but he was too lenient in believing that there's no hole in High Lord Rashal's ranks. His focus was narrowed, fixating on simply the Grey Realm and forgetting that there's more out there that could be aimed.
And more than that, there are also Gods that might dare to intervene.
I'm sure the Overseer isn't going to allow any God to tamper with his domain, but it needs to exceed a certain limit. In this case, the enemy wasn't aiming to destroy the Cavity, but damage it enough to shatter High Lord Rashal's reliability.
