I got mine from killing a taken mini boss. When you see taken, start killing them, and a couple of waves will form with the final one having a mini boss such as a yellow knight. Not sure if you’re guaranteed a rune for the first drop or not though. The day I got mine was from a group in the big room where you start patrolling. Apparently groups form in different spots every day. Once you get the rune , it will take a week to finish because you’ll have to find and kill the group everyday.