Thanatos reworks the death system in a multiplayer-focused way. When a player dies, they are not presented with the death screen. Instead, they become a ghost that cannot interact with the world until they respawn using a keybind. There are several mechanics that interact with this system, such as the ability to see ghosts, possession, and the ability to temporarily prevent ghosts from respawning.
The source code is available in the sources jar attached to each version.