The deathly usher drops a rune of ivy needed for the ranger Ivy Etched Helm quest. I am still looking for the Rune of Ivy but I can give my experience trying to hunt these guys at various levels
between 20-46. The information here says level 29-31 and that seems accurate to me.
At level 20 I could barely solo the entrance area so I did that for a few levels and came back.
I tried again at level 30 but even though camouflage can let you hunt them down, there is no good spot to pick them off.
So I came back at 40 and I found that they spawn in the upper levels. Especially in the main upper chamber (I still haven't explored that) of the castle. But I have seen them in the side rooms on the upper level. At level 40-46 I still have trouble with the upper rooms due to the number of mobs. The Ushers also spawn below that, in the larger chamber on the entrance-level of the castle if you go straight in.
As mentioned in a comment here and even on the Atlas, the Usher DOES spawn in the jail underneath the graveyard, which you can reach by going inside the tomb and then falling down a hidden trap (which takes off invisibility). There is another secret entrance in the hillside wall on the left side of the tomb. The Usher does not always spawn in the jail. Often it is will sappers. There is also a wandering mob who shows up -- so that is 4 total with the 3 in the jail cells.
By the way, the jail cells no longer lock. Each individual cell can be opened from the outside and the inside on the live server. Still, all 3 inside the jail cells will come after you.
Good luck!