Not sure what you want exactly so I'll give two answers:
Check out:
CEntity
Also you should read this:
https://forums.alliedmods.net/showthread.php?t=283518
Edit: I've made "Custom" entities with CEntity, check out
https://youtu.be/nLySY7d5Jws?t=1m25s, which custom trigger brush entity, logic ect... But everything is in C++ and I don't use any plugins. You also can't use GetEntProp to look up your custom properties.