#irl bug

you get A SILVERFISH!

In universe, there's no real way to know

Maybe it was the only unoccupied space large enough to house the egg

Maybe the egg or the chamber at its core was already there, like the weird chamber where the shade spawns in the colosseum of fools, and it was later repurposed for the seal

Maybe the location has relation with how they sealed Radiance in the first place

But the location is rather unassuming, being pretty much a commonly transited route not that close to crystal peak or in some sort of center

Not really helpful to dispell the mystery, but I personally like to think/headcanon that the corpse was that of an ancient caterpillar (Bardoon shows us caterpillars can get really large in this world). Some of the soul totems seem to depict caterpillars as well, famous example being the dragonhead caterpillar.

The horns all over the temple makes me think strongly of this one (I search it up under the name of dragon caterpillar (not to confuse with dragonhead) but honestly I'm not sure if that's the actual name)

The Little Mesquite Cicada (most likely)

Female

GASPED SO LOUD, these guys look so awesome, they're like if ladybug larvae had a second form

Reading up on them some more it seems that these guys have crazy dimorphism going on. Apparently the females remain in this more larvae looking stage hence the trilobite look while the males look vastly different.