Athena Event-Engine is a project that aims to create a general system events for all kind of L2J servers.
That means if you have an official L2J server or a custom fork, the engine would work without any change on the events code.
The code was based on L2J Event-Engine made by fissban, U3Games, Zephyr and Sacrifice78.
For the detailed information about building and developing Athena Event-Engine, please visit the forum. This page only gives very basic information.