While Arad flew out with the drakainas out of the Rainbow Mountain, his other incarnation was still flying through Euri's world, consuming everything in his path.
The more he ate, the bigger Eris's world grew, and the bigger Eris's world became, the easier it was for the angels to get the materials necessary to build her palace. And with the palace taking shape, those angels were able to draw the divine spells on its wall, allowing Eris a bit of respite as her divine duties were automated.
The building process was so fast that it was scary- the angels easily twisted the fabric of her sun and bent steel with their bare hands, raising great walls, sprawling gardens, and everlasting pools of roiling plasma.
It would've taken much longer, but Eris was lucky. She had Arad, who could easily provide all the building materials she needed, and her angels weren't newborns- they were old, experienced, and powerful.
