Hungry Ghost
Hungry Ghost is a perk that summons a green ghost that follows you around. The ghost will occasionally eat projectiles shot towards it, with each eaten shot providing you with a 
Damage plus small temporary status effect. Each ghost can eat a projectile it finds in 12 pixel radius, as long as it's not owned by the player, every 160 frames.
Stacking status effects
This Damage plus small damage boost is an extra modifier (a special type of modifier that gets applied after every Projectile, Static Projectile, or Material type spell you cast) that adds 7.5 
Projectile damage, then multiplies all added damage by 1.25. This can lead to extremely high damage amounts if you have a large number of spells with valid types or many stacks of the buff, as your damage bonus will end up looking like etc., which is a form of exponential growth. This can be used to obtain infinite damage, see Extra modifiers#Guide to infinite damage for details. Hungry ghost has a delay of 160 frames (~2.67s).
Warning
The Hungry Ghost(s) can be extremely deadly in some circumstances. For example, if you're using a teleport healing wand build, and in the middle of an intense fight the Ghost gets supercharged eating a rich number of projectiles (or if you intentionally use something like the Magic Guard spell to power them up), all the stacked damage boost will be applied to the healing spell as well, which will then instakill you during the next teleport.
This CAN however, be nullified using the Null Shot spell, however only if one is using Circle of Vigour or Deadly Heal, as those apply a regeneration buff, as opposed to Healing Bolt which heals you for a fixed amount.
Transformation
Collecting Hungry Ghost, Angry Ghost, and Mournful Spirit will apply the Ghostly Transformation which gives you a ghost companion and increases your levitation time by 15%.
