Unfortunately, the Horror was kept nearby an enormous coal seam.
Of course, people thought it would be perfectly fine to dig a mine in proximity to such uncontrollable and unpredictable danger. And of course, as soon as the Horror wakened, it felt the proximity of mortals. Rather than continue to fight the gods outright when it was still groggy and starved, it pierced a passage between its prison and the mine and immediately devoured all those who were working in the depth.
The gods reacted as quickly as they could, but the underground is a difficult place to be for everyone that is not an Eldritch Horror. Some mineral compositions react very poorly to some types of magic. Fighting down there would have made the gods risk their own life. And
that was obviously inconsiderable.
Instead, the
architecture god collapsed the mine on top of the Horror, and the
forging god set fire to the coal seam. You might think "a little bit of fire" is not good enough to keep in check an Eldritch Horror, but the ensuing explosion was felt even in the capital! Trapped on all sides with eternally raging fire, buried deep underground, there has been very little the Horror has been able to do to harm anyone.
Creator of Araea, Megacorpolis, and many others.