Nightmare of Mensis
Make additional discoveries.
Found near the Mergo's Loft: Base lantern. Follow the path to encounter a few boulder-throwers along the cliff. A pair of them guards this Rune.
Acquire more Blood Echoes.
You can find this during or after the battle with Micolash, Host of the Nightmare. When you reach the large staircase, head down. At the very bottom, past all the baddies, is a corpse holding the Rune.