As the description says, being cleansed is a requirement to be able to transport the Eye of Janthir in the Divinity Coast mission. At least one of the party members must be cleansed so that the party can proceed with the mission. The Eye will follow the cleansed party member closely. If more than one member has been cleansed, the Eye will occasionally switch between them. The following tips can be given about being cleansed:
If a frontline member (such as a Warrior) is cleansed, the party will greatly benefit from the periodic knockdown effect of the Eye during combat situations.
If a backline member (such as a caster or a Ranger) is cleansed, it will be easier for the party to protect the Chosen since they tend to follow the Eye.
If most or all of the members are cleansed, the Eye will constantly move from one member to another, which can be distracting for some people.
Note: It has been confirmed that Heroes and henchmen can be Cleansed as well. A blue-ish aura will appear on them. Pets can also be cleansed. The eye only follows human players.