Phantoms have the look of translucent giant squids floating through the air. They roam portions of the Ring of Fire Islands in small groups but sometimes also appear solitary out of thin air. When defeated they vanish into nothing, without leaving a corpse behind. They neither deal nor receive much damage, but their ability to blind and interrupt can become very annoying.
Beware that their AI is capable of interrupting even the quickest of skills. This can be used to your advantage, however. Cast meaningless spells first (cheap spells, preferably ones with short recharge times) and save important spells for later.
Try to avoid clumping casters together so that Cry of Frustration only interrupts the target and nobody else.