Monks of the Way of the Waning Moon follow a tradition that values the natural world and achieves self-transformation through meditation and understanding.
Starting when you choose this tradition at 3rd level, you gain the ability to mystically change your shape into that of a beast. You select one such shape, belonging to a beast which you have seen before with CR no greater than 1, and gain a deep familiarity with it through an overnight ritual. You may shift into this form at any time as an action, and out of it as a bonus action. You may additionally shift into or out of the form as a free action any time you spend one or more ki points on another ability.
The form you select must have Strength, Dexterity, and Constitution that total to no more than yours do (you may reduce those ability scores as needed to match your own). Initially, you may not use any other form of movement that it might possess, such as swimming, flying, or burrowing speeds. Your natural weapons and attacks count as 'monk weapons', but you may not use your Extra Attack class feature while in bestial form. For the purposes of your Martial Arts feature, any Action that performs an attack counts as "the Attack Action".
You may perform an 8-hour ritual at any time to select a new form, which must be performed in a setting appropriate to the form being selected.
While you are transformed, the following rules apply:
Beginning at 6th level, you may use your bonus action to spend up to 3 ki while in a beast form; roll that many d8s and gain temporary hit-points equal to the total.
Your natural weapons count as magical for the purposes of overcoming resistance and immunity to nonmagical attacks and damage.
When you reach 11th level, you may chose a second form into which you can shift; it must obey the same CR restrictions, but it may have swim or fly speeds, and it may be an Earth, Air, Fire, or Water Elemental.
You may additionally cast Speak with Animals or Speak with Plants as an action by spending 1 ki.
Starting when you reach 17th level, you may choose a third form into which you can shift; it must obey the same restrictions as the second form. If you are reduced to zero Hit Points while in the shape of a beast, you may spend your reaction and pay 4 ki to immediately shift into another form. You may not use this feature again until you have taken a short or long rest.
Additionally, when you cast Speak with Animals or Speak with Plants on yourself, its duration is extended to 'permanent', though it can still be dispelled or nullified.