"It looks like CluDathoon has summoned the Wither Queen, and is allowing it to gather strength. The Wither Queen is one of CluDathoon's strongest commanders, and we must strike at her before she reaches full power. This is not a solo endeavor! You should assemble a team of Guardians, and strike at her hideout in the Cavers. Your Compass will guide you."
Outside Nichol's house, head up the stairs leading to the caverns. Follow your compass to the hideout, using one of the bridges to get to the opposite end. Before entering, make sure you have at least 5 Guardians to fight the Wither Queen. When you have done so, go inside and walk up the steps. You will summon her when you reach the top.
When fighting the Wither Queen, make sure you have at least one Guardian on each of the Healing Crystals, while the rest focus on the her. After your team has defeated her, wait until you have been teleported to the loot room and have collected your loot before heading back to /warp chapter2.