Phasm

This creature looks like a slithering blob of multicolored goo perhaps 5 feet wide and a little less than half that high.

A phasm is an amorphous creature that can assume the guise of almost any other creature or object.

Its shapeshifting ability frees a phasm from most material needs, which usually leads it to a life of exploration, hedonism, or philosophical contemplation. There’s no telling where a phasm will turn up, nor what it will do if discovered. They are natural spies but notoriously unreliable, since they feel no particular need to report what they learn. They have an affinity for doppelgangers and sometimes ally themselves with groups of them for security⁠—​or just for the fun of it.

A phasm in its natural form is about 5 feet in diameter and 2 feet high at the center. Swirls of color indicate sensory organs. In this form, a phasm slithers about like an ooze and can attack with a pseudopod. It weighs about 400 pounds.

Phasms can speak Common but prefer telepathic communication.

Combat

When faced with potential danger, a phasm is equally likely to retreat, parley, or attack, as its fancy strikes. Phasms value new experiences: fresh scents and tastes, obscure facts, gossip, odd bric-a-brac, and the like. Those who offer a phasm such things stand a good chance of avoiding a fight.

If pursued or harassed, a phasm transforms into the most fearsome creature it knows, such as an adult white dragon or a fire giant, and attacks. When seriously hurt, it changes to some fast or agile form and tries to escape.

Amorphous (Ex): A phasm in its natural form has immunity to poison, sleep, paralysis, polymorph, and stunning effects. It is not subject to critical hits and, having no clear front or back, cannot be flanked.

Resilient (Ex): A phasm has a +4 racial bonus on Fortitude and Reflex saves (included in its statistics block above).

Alternate Form (Su): A phasm can assume any form up to one size category larger than its natural form as a standard action. A phasm can remain in its alternate form until it chooses to assume a new one or return to its natural form.

Tremorsense (Ex): A phasm can automatically sense the location of anything within 60 feet that is in contact with the ground, so long as it is touching the ground itself.

Skills: When using alternate form, a phasm gains a +10 circumstance bonus on Disguise checks.

Source: Monster Manual (version 3.5), page 208. Changed the Advancement entry to reflect a more orderly progression from Medium size to Gargantuan size. Changed the Alternate Form entry text “A phasm can assume any form of Large size or smaller as a standard action” to “A phasm can assume any form up to one size category larger than its natural form as a standard action”. The rest of that entry is corrected for errata, with a hyperlink added to the alternate form special ability, which is also corrected for errata.