Eligor's plan was simple.
Attack the entrances that were used by Agarthians in order to enter their underground Kingdom.
He had already gained information from a King of a Kingdom that was affiliated with Agartha.
Of course, the King didn't give away this information on his own accord.
That would've been impossible.
Eligor extracted this information directly from the King's Soul, reading all of his memories like reading a book. This was also why he had proposed that all Demigods temporarily withdraw their offensive against the Dragons and the Elves, shifting the focus on Agartha.
He believed that it would be easier to capture the Ancient Kingdom, which only had three Supremes as its Guardians.
Aside from King Septimius, there were two more Supremes that protected Agartha, keeping it safe from outsiders.
Still, the Fallen Angel wasn't worried. Their side had fifty Demigods who would join him in this invasion.