Fragment of Anguish

The Fragment of Anguish is a keystone used by the to allow entry through the magical barrier of Anguish Fortress, while keeping it sealed to outsiders at the same time. Former elder Rulkan kept at least one when her exiles broke away from the clan.

Due to the deep respect she shared with Prophet Velen, Rulkan gave her fragment to the Alliance commander sent by Velen in order to help the draenei people and stop.

Source
There are two versions given for the quests Darkest Night and Into Anguish.