Hold and use the manual of chain lightning that the An ettin cannibal drops.
Requires skill in chain lightning be blow 65%. Requires Archon level be 70 or more.
June/15 2019: Might be different now however. Below is just wearing the manual and not using it. Chain lightning skill after was still at 65%.
You hold a manual of 'chain lightning' in your left hand. For besting a savage ettin cannibal in mortal combat, the Powers have granted thee a mark! You have gained the Mark of Vengeance! You gain 50 archon points! You gain 75 practices! A manual of 'chain lightning' vanishes from your grasp without a trace.