Thanks for the detailed bug report! Obviously the error message would have been good to have but I can get a pretty clear picture from your description and the exception dump. Since you mention "piercing damage cascade" I suspect that you had the Windrider's Bow equipped. Do you remember which other relics you were using?
Yup yup, the bow. There was no error message from the game, it died with the dump report and that was it.
Other items: Preserver, Belt of Balance, Moonstone Essence. Oh and demonologist iirc.
None of which shouldn't have impacted anything, as far as I know.
It was right at the end of a wave, though, last enemy (except the cosmic horror) dead. So it might've been an issue of timing between changing waves and dealing damage to the horror.