The vendor named Grizzby (Ratchet) will give you tasks to bring him items (see below). Fetch those items and you will be able to purchase the rune from Grizzby for 5 Gold.

This ability benefits from and triggers all effects associated with Heroic Strike. 20 Rage Head to Ratchet and talk to Kilxx by the docks. Purchase a Fishing Harpoon and head into the bay and find ...