The Guild Hall is a building accessed via the library. It is unlocked by entering the code "9853" into the secret door using [W].
Within the Guild Hall is Lycan, whom you can talk to [T] and can give you a quest to retrieve 3 Wulf Essences by eliminating the three turncoats . They will appear randomly as you explore areas and you will be given the choice to engage or run away before the fight begins. The turncoats themselves scale to your level and typically have lock and/or thorning along with other affixes. Once you defeat the three turncoats you will be given an upgraded version of Riposte called Ravage, which will increase damage riposted to the enemy.