The EGG file extension is related to the Explosion Graphics Generator. The *.egg file contains script.
EGG file extension - Explosion Graphics Generator script file
What is egg file? How to open egg files?
File type specification:
The EGG file extension is also related to Explosion Graphics Generator. This is a simple scripting language for implementing particle system graphics effects, which I wrote in September 1998, updated in June 1999 to work under Windows using MSVC, and updated again in July 1999 to work under Linux. It is not at all optimised and therefore runs quite slowly, so it isn't suitable for realtime use, but it may be useful for making prerendered animations, as a testbed for designing effects that you will later reimplement in more efficient native C code, or as a library that can be linked into your program and used to generate some graphics when the program initialises.