Really sorry about this! I'll attempt to fix it as soon as I can though to quickly ask, what is attacking the terrorbeak? since normally mobs wouldn't attack nightmare creatures.

This is the bug I've been getting. Here is the end of the log. https://pastebin.com/7H4ehrCa

The 2nd terrorbeak attacks a mob first then the mob attack him back iirc, since he roams around he'll die eventually then the game crashes

I am experiencing this bug as well. After a certain day, every time the Nightmare dies, it appears to spawn a new neutral Terrorbeak upon exiting and reloading the server. I have had up to three neutral Terrorbeaks wandering aimlessly around the screen at once. They will randomly engage neutral mobs, such as Koalefants, Beefalo, even birds, and when one dies I get a game crash with an error message stating an error that is something related to the Terrorbeak.
Especially because he really gets in the way if you're trying to build or plant, I would really second the idea for the Terrorbeak to be configurable.

This is one of my favourite modded characters but eventually, all my servers with him become unplayable because inevitably the neutral Terrorbeak will engage something and crash the server.

this has happened to me as well for a good while now and i only just figured it out like two months ago. nightmare is literally my favorite character, and i think this only became an issue after a gen game update. it's definitely an issue if you're near the other TB when mobs (like agro beefalo) go after it. i think its just a game processing problem, but it def sucks.