Quest ransack

Quest Ransack, as introduced in Update 19, is a stacking penalty for repeatedly playing the same quest. You may play each difficulty level once per character, per incarnation without incurring Quest Ransack penalties. After that, each playthrough will add a 20% stacking penalty to base XP, up to a maximum of -80%. Every day the penalty is reduced by 50%.

See also;
 * Lamannia forum official thread
 * Update 19 Release Notes