MultiOgarII is a FOSS agar.io server implementation that is based upon the work done by Barbosik.
Before the server can be used you need to install NodeJS onto your machine. Instructions for which are available on the NodeJS website linked above.
The server can be ran by executing the following commands in a Linux terminal or a Windows Command Prompt. This is assuming that you're in the root of the project directory.
# install modules
$ npm i
# change directory
$ cd src
# start server
$ node index.js