Tremors in the Stone Diablo III Guide
Last update: 11 May 2016
Since Tyrael can't be found anywhere in the Bastion's Keep Stronghold you must speak with Lieutenant Lavail to begin this very short mission.
Go to the Armory
As the objective states, you must travel to the Armory.
Kill the Shadow Vermin
Leah will lose control over the Black Soulstone for a while and a dozen of Shadow Vermin will appear in the Armory. Thankfully they're not a serious threat and they're very easy to kill.
Talk to Leah
Talk to Leah to officially end this quest.