White Walker GoT NPCs

Hello, I’m new to modding so it’s a crash course. I am working on making a Game of Thrones - White Walker mod for our GoT server. The goal is to have these walk around in the snow area of the map and aggro to everything and be unkillable except with fire or obsidian arrows. Any other weapon will give them a 2 second KO (torpor) or stun. Eventually, I would add the Night King as a boss type NPC. This is my first mod so I’m sure I’ll have a ton of questions.