Cave Story is a game released in 2004 by Daisuke Amaya, better known as Studio Pixel.
In 2019, CuckyDev ported CSE2 to the Wii, a decompilation of Cave Story's freeware version.
This is an updated version of the port, with slight changes (such as a pause menu, and certain files being external to allow semi modding support) to make the experience better.
As well, there's now local co-op plus many mods built-into the port by default.
This includes the Lua scripting API, "ModCS", although it had to be changed significantly to support multiplayer.
There is no documentation for the Lua, or modding API currently.