For this farm, you will want to head to Terokkar Forest, and enter the Ring of Observance. You will want the northern portal Mana Tombs.
I wasn’t to fond of this location, but for those who enjoy farming dungeons this may be for you. One run took a little under 4 minutes and I was able to get 7 motes of shadow, with there being a 10 run limit, you can only get around 5-10 primals per lockout.