Finding Avalon: The Quest of a Chaosbringer summary:
The main character loves the MMORPG game called Dan-Ex. During one
particular day, a beta test event was announced, and he managed to beat it.
In exchange for clearing the event, he was granted permission to try the beta test server.
Forcing to choose different avatar and skill as the system doesn't allow his own, he eventually settled on random. However, he is transferred to a place where, as expected, he finds a fat, villainous mob character...!