Hrothmund's Blessing may be granted by the spirit of Hrothmund the Red, the first chieftain of Thirsk, when attempting to pick up his axe. This may be performed at the request of Svenja Snow-Song during The Mead Hall Massacre quest.
The axe is found inside Hrothmund's Barrow, which is sealed and requires the Nerevarine to utter the name of the Fell Wolf that killed Hrothmund, Ondjage, to gain entry. This can be learned from Louis Beauchamp during The Patchwork Airship quest.
This spell is not available for purchase and cannot be learned during regular gameplay. Although not intended to be cast by the Nerevarine, it can be acquired on the PC by using the console:
Player -> AddSpell "hrothmund blessing"