OnlineKill

Just Another ULTRAKILL Multiplayer Mod

About The Project

Experimental multiplayer for ULTRAKILL, built around a small dedicated UDP server instead of Steam lobbies or Steam P2P.

Current State

This is the first playable slice:

• BepInEx plugin for ULTRAKILL.
• Standalone OnlineKill.Server executable.
• Manual IP/port connection through the in-game F7 menu.
• Basic local player position and rotation sync.
• Remote players shown as simple capsule stand-ins with name tags.

It does not sync weapons, enemies, damage, doors, checkpoints, Cyber Grind, or level state yet. Those systems should be ported in stages from the JAKET/COAT/Polarite-style references onto this transport layer.

Build

dotnet build .\OnlineKill.sln -c Release

Run a Server

dotnet run --project .\OnlineKill.Server -- 27040

For internet play, forward UDP 27040 to the machine running the server.

Install the Plugin

Copy OnlineKill.Mod\bin\Release\netstandard2.1\OnlineKill.dll into BepInEx\plugins.

In game:

1. Press F7.
2. Enter the server IP, port, and your player name.
3. Press Connect.

Why Not Use MultiplayerUtil?

MultiplayerUtil is useful, but its documentation and XML comments show it is built around Steam lobbies and Steam P2P. OnlineKill uses direct UDP because the goal here is a dedicated server that can run without Steam networking.

Warning

This is a work in progress and is not yet ready for public use. It is not recommended to use this mod in its current state. (also uh, its vibecoded lol)