Enter the phrase: j94esxih under "search by Chalice Glyph" to get to this Chalice Dungeon. The second layer boss should be Keeper Of The Old Lords. Once you defeat him you'll be able to purchase the set from the insight vendor in The Hunter's Dream.
If you can't get into that exact dungeon try making a Pthumerian Root chalice and you should run into this boss in the center section as the second boss fight.
