Finally touching the hollow after adding it. This is the hollow's equivalent of the warden: The Infector.
It is slow but can annihilate you in literally two seconds if you have no armor (although it's far weaker than its sculk counterpart), and as its name suggests it can infect nearby mobs (including tamed mobs, certain mobs are immune), turning them into enemies.
The terror bird is a large flightless raptor that spawns in savannas and jungles and primarily hunts mid-sized mammals, they are mostly solitary but occasionally form groups to harass saber-tooth prides or to impress potential mates.
While they attack you on sight, terror birds can potentially be tamed by sneaking up on them and wrestling them into submission. Beware that if they manage to buck you off then you will immediately get mauled.
In real life, the terror birds were a family of birds (phorusrhacidae) closely related to the extant seriema, with the species in PrimalCraft being Kelenken guillermoi, one of the largest it lived 15 million years ago. Terror birds were the apex predators on south america during their time, although they shared this title with crocodilians and teratorns (close relatives of vultures and eagles).
A couple months ago Mojang did a big oopsie (which they have yet to fix and I am beginning to doubt they ever will) that broke experimental features and AzhdarchiBlock was taking like a whole day to upload for each update because of how much storage the audio files took, it's actually why I was making arachnophobia.
Anyway AzhdarchiBlock has been banished to the depths of the underworld and I'm working on a new mod to replace it called PrimalCraft, some content from AzhdarchiBlock will also be in PrimalCraft because their models fit there.
I'll edit my masterpost to include PrimalCraft and a list of its content