To unlock this you just have to beat the final wave of a Horde mode. Most of the ones off the start are pretty easy to complete and are only 2 to 3 waves. The one that gives a Butcher Pin as a reward is quite a bit harder than the others at the start.
If you search for a clan with the name "horde" you can find many clans named a variation of "Horde and Go" or "Horde N Go". The way these work is people just join the clans and use their thumpers and leave. This makes it easier to find active people at the same time as you are on.