Great Devourer

The Great Devourer is a piece of Pathfinder munchkinry analogous to 3.5's Pun-Pun.

Details

The exploit hinges on the fact that the first-level spell "Summon Minor Monster", intended as an alternative to the usual Summon Monster I that summons 1d3 almost useless creatures instead of one weak creature, doesn't have a list of creatures it can summon. All other Summon Monster spells have a table listing the creatures that can be summoned with that spell, but Summon Minor Monster can summon any tiny animal, with the listed creatures being examples rather than an exhaustive list.

Usually, this isn't a big deal; pretty much every creature listed as having the "tiny" size and "animal" type are nearly useless. However, the spell doesn't actually summon normal creatures, but beings with the Celestial or Infernal template, from Pathfinder's equivalents of Heaven and Hell. (Which are apparently filled with demonic/angelic versions of ordinary creatures, although this isn't mentioned in most descriptions of them.)

Since these Outer Planes are repeatedly referred to as "infinite" (the exploit argues), they must contain every possible variation on these creatures somewhere within them - young creatures, injured creatures, creatures under various different spell effects etc. As long as they're still "tiny animals", they can therefore still be summoned by Summon Minor Monster. One possible variation would be to apply different templates. Some templates would conflict with the Celestial or Infernal template (requiring the creature to be Neutral, for example) or change their type to something other than "animal". Others, however, are much more reasonable, like summoning a younger or older version of a creature (as long as it's Tiny) or summoning a version that's unusually experienced.

There's a specific template - Consuming Creature - that represents an animal that has learned to eat other creatures and gain their powers. There's no limit on how many powers can be gained in this way.

As an added benefit, the Celestial and Infernal templates grant increasing levels of durability the higher the creature's Challenge Rating, and Consuming Creature increases the target's Challenge Rating the more creatures they consume, making the Great Devourer nearly invulnerable to normal attacks.

However

Consuming Creature is not a monster, but a template. Summoning spells are usually assumed to only summon basic variants of monster unless specified otherwise. This is done exactly to block such shenanigans by artificially upping a monster's CR by stacking loads of templates onto it to make it way more powerful than befits its level.

Edit: This is also a 3rd party template, so y'know.