↩ lyicx The mod I used (Custom NPCs) has a a few restrictions and stuff works weirdly at times (The thing got stuck on hills so often I had to rerecord several times).
I actually wanted to make custom sounds but never got a chance to so I winged it with Minecraft sounds. I'm up for updating this though. I had plenty of ideas which didn't work out cause it either complicates the set up process, one of them being making this entity the head and another entity the body, but that would require summoning two NPCs and making the head ride the body NPC. This mod also doesn't have a way to make spawn eggs or summoning systems without already having it spawned it, but you can set natural spawns so it spawns like a mob. A NPC also doesn't chase you down if it's set to shoot projectiles and the main idea behind this boss is the slow paced chasing and the nausea it deals.
I might just go through with the idea I had about having it be two entities stacked together which could lead to a nice effect of it being alive with missing parts (ignoring the set up needed). That would also let it shoot and chase at the same time with the head shooting and the body chasing. This mod is honestly amazing if you explore it's possibilities.
If I knew how to code mods, I'd do it in a heartbeat. But I can share the file for this NPC so anyone with the mod on 1.12.2 can use it on their world.