Avenger is the Dark side form of Warrior. Normally, you would need to select from a list of Dark Side classes in Character Creation, but the servers with this Class have just modified their files to allow them to let people manually job change to it from a Warrior instead of Sword Master. That is why you see people with Avenger/Moon Lord - because it isn't fully implemented.
A lot of effects and functions of the Avenger are only partially working as well. There is also no Dark Avenger sub class yet.
It's a lot easier for them to add in an extra Warrior class rather than an entire new class (Lencea) which requires newer files that very few people have. They don't have the newest files - only the ones that had the modded Avenger.