The Dark crystal (melee weakness) is one of the debuff crystals dropped by bosses during Morvran's Slayer challenge. It is not dropped by any specific monster. The crystal increases all melee damage taken by 25%.
When attempting to destroy any dark crystal, the player will not be able to progress in Morvran's Slayer challenge unless they are carrying all dark crystals earned so far while killing the next boss. Destroying the crystal will make the player have to reclaim it from Morvran to continue in the challenge.