The Forgotten Server is a free and open-source MMORPG server emulator written in C++. It is a fork of the OpenTibia Server project. To connect to the server, you can use OTClient or OpenTibiaUnity.
$ apt-get install git cmake build-essential liblua5.2-dev libgmp3-dev libmariadb-dev-compat libboost-filesystem-dev libboost-system-dev libboost-iostreams-dev libpugixml-dev libcrypto++-dev
$ mkdir build && cd build
$ cmake ..
$ make -j4