The Ram's Rune
is located in the Ossuary
. You access the Ossuary
from the Promenade of the Condemned
. The entrance is always located at the far right of the map. The Teleportation Rune
is required to get there.
Once you have the Teleportation Rune, you need to take this path:
Prisoners' Quarters > Promenade of the Condemned > Ossuary
I found this map to be very helpful.