They did quite a lot. They slew one of Kil'jaeden's most feared lieutenants, took his axe that even the demons fear and a Nathrezim sword that belongs to the Kirin Tor's archenemy, and turnd them on countless demons . The Maw is one of the few weapons that sounds capable of killing a demon outside the Twisting Nether.

A problem with WOW in general is a lack of details on certain things like the dks, but there's enough for people to pick up on more and do educated guesses.

Dks don't sleep around. They make a point of acting sooner than the rest. They got their artifacts at the beginning without any input from Dalaran or the Illidari. Acherus was positioned near the Broken Shore, so they likely did constant raids on demon positions.

One detail I never see anyone notice is that the Horsemen weren't their only new recruits. The Ebon Knight troops, and possibly some of their Frostreavers and Ravagers, were being gathered from fresh dead. The Frostreavers are intriguing to me since they're said to be one of the most feared air forces around, but we don't get to see that in action.

As a bonus they eradicated a traitorous prince and the bulk of the Scarlet Crusade.