Found in the docked Reaver ship inside the Northri Stronghold (Favor: Family Business), beside a knight corpse
Dock at the Mason’s Channe beachl (or just take the lift in Helheim Tower from Tyr’s Temple) Climb the ledge beside the one of the oarsmen statue as shown below then shimmy across the gap.
Pass by Sindri’s shop then climb the next cliff. Enter the dark caver to your immediate right which will lead to an outcrop beside the oarsmen statue. The mound is located near the statue’s hand, by the edge of the outcrop.