This spell transforms a creature that you can see within range into a new form. An unwilling creature must make a Wisdom saving throw to avoid the effect. The spell has no effect on a shapechanger or a creature with 0 hit points.
The transformation lasts for the duration, or until the target drops to 0 hit points or dies. The new form can be any beast whose challenge rating is equal to or less than the target's (or the target's level, if it doesn't have a challenge rating). The target's game statistics, including mental ability scores, are replaced by the statistics of the chosen beast. It retains its alignment and personality.
The target assumes the hit points of its new form. When it reverts to its normal form, the creature returns to the number of hit points it had before it transformed. If it reverts as a result of dropping to 0 hit points, any excess damage carries over to its normal form. As long as the excess damage doesn't reduce the creature's normal form to 0 hit points, it isn't knocked unconscious.
The creature is limited in the actions it can perform by the nature of its new form, and it can't speak, cast spells, or take any other action that requires hands or speech.
The target's gear melds into the new form. The creature can't activate, use, wield, or otherwise benefit from any of its equipment.
* - (a caterpillar cocoon)
Is there any CR limit of what you can turn yourself into? My idea is to transform into a high ranking demon that can fly.
"The new form can be any beast whose challenge rating is equal to or less than the target's (or the target's level, if it doesn't have a challenge rating)."
Vallid polymorph-into options must be:
So no polying into a demon.
is an object a creature? Or is an object an object?
Cmon.
Let's bring this all together.
The spell description answers a lot of the questions that have been raised.
Would a monk polymorphed into a giant ape still have monk abilities?
No. All of the monk's game stats are replaced by those of a giant ape.
Could a caster maintain concentration after casting the spell on themselves?
Possibly. Since the spell was already cast, losing the spellcasting ability wouldn't end the spell. Detect thoughts implies that a creature with Int 3 or less has no thoughts. If the beast form has Int 3 or less, it's likely it wouldn't be able to keep concentration. Higher Int, however, could easily still be capable of concentrating on the spell, using the Con of its new form to make saves if it takes damage.
Can power word kill be used to kill a polymorphed creature?
Yes, but it only kills the polymorphed form. Here's the order of operations.
The general rule that dead creatures can't regain hit points prevents effects such as cure wounds from restoring the dead by adding hit points. In the case of polymorph, specific rules override general. The dead creature isn't truly regaining hit points. It's returning to its alive, pre-polymorphed state.
Can a fiend warlock eventually cast polymorph?
If you take the Sculptor of Flesh invocation you can cast it once per long rest using a spell slot. Otherwise, it's not on the Warlock spell list.