Kill in Stormhold until a mob drops Sword of Thunder (Note, Sword of Thunder is LORE but tradeable. So you could purchase it off the broker.)
IMPORTANT: According to rumors, the Sword of Thunder is a chest drop AND a body drop. This means it can drop of a greyed out mob. However, this I'm not 100% sure of so it is probably a good idea to mentor down to 19-21 and look for it. *Confirmed* Dropped for a solo level 70, chest drop, Permafrost
Confirmed* Drop for solo lvl 55, chest drop from final Bone Claymore Kill of Moltar.
Examine the sword and it will tell you to kill 100 undead knights in stormhold. Only undead count. Once this step is done talk to the sword again.
NOTE: Do NOT accept the quest until you complete the dialog. If you do, the quest may become bugged. Once you accept the quest an item will be put in your inventory called "Berik, Sword of Thunder" This is the sword you will need to inspect after each step.
Next he will ask you to kill 3 named bosses in Stormhold: