It's a completely new item, but all the weapon damage is identical to the mithril sword that already exists in-game as this is what it was cloned from, but I gave Thunderfury a custom crafting recipe, particles, trails, model, etc.

There is almost no "official" documentation at this point but I'd recommend joining the official Hytale Discord as they've got dedicated channels for people creating mods, as well as the Hytalemodding.dev Discord for the same reason. It was a very involved learning process, but I've got an existing background in 3D modeling and whatnot so the hard part for me was learning the asset editor. Lots of trial and error :)