This map was originally released the 19th of July and launched the modding adventure in Mordhau. I'll copy here a part of the announcement made a few days after on the modding discord :
Plou worked on a way to create maps before the SDK for weeks, and once he managed to do so, he created the Berzerker Arena as a prototype map, that wasn't meant to be shared. We kept this on the down low and tested the map from time to time on a private server. The map eventually became good enough to be a viable duelyard. Before using it permanently on one of our servers, he had a talk with the devs, explaining his work and making sure we would'nt get into any trouble for doing this. Turned out not only they were cool about it, but they also gave us the greenlight to share the map & the way to create more, and create a discord around mapping and modding.
Recently someone asked for it to be on mod.io so here it is, keep in mind that it was done on a different engine version, and without all the features that have been made possible later on, such as multiple spawns, character selection screen placement, and built lighting.
I'm not the creator but since Plou gave me the uncooked files i might port it to UE 4.20.3 in the future and add these features but for now this is the original files.
The level name is BerzerkerArena (no prefix)