This Library/Interpreter is used to enable the CustomNPCs EMCAScript. If you are running the latest version of Java chances are you are already able to script NPCs once you enable EMCAScripts on the NPC. However, if you are attempting to script an NPC and it does not allow you to do so, chances are you are missing this Library/Interpreter. The details of this are listed below. I did not create this, I simply ported it to Curse for Modpack Creators. It makes it easier for creating a modpack. This is ALLOWED by terms of this Library/Interpreters licence. http://openjdk.java.net/legal/gplv2+ce.html
As per the licence requirements I have also set this project's licence to use the GNU General Public License version 2 (GPLv2)
This does not do anything on its own. Quite literally CustomNPC hooks into it and "turns on" the EMCAScript engine. If other mods implement a Nashorn Engine they will hook into this as well. Personally, I am not aware of any, but there may be some out there.
jrunscript command-line tool.
InvokeDynamic APIs described in JSR-292.