Starting when you choose this path at 3rd level, you gain the ability to magically change your shape into that of a ferocious beast. You select one such shape, belonging to a beast which you have seen before, and gain a deep familiarity with it through an overnight ritual. You may shift into and out of this form at any time as an action, or as a free action when you begin raging.
The form you select must have Strength, Dexterity, and Constitution that total to no more than yours do. Initially, you may not use any other form of movement that it might possess, such as swimming, flying, or burrowing speeds.
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:
After reaching level 6, your unarmed attacks or attacks with natural weapons (such as are used by your forms) deal magic damage, and critically strike on a 19-20.
Starting at level 10, you may choose an Elemental form alongside your Beast form when you perform the ritual (it must be performed for each new or changed form). This second form has separate Hit Points and Hit Dice, and you can enter one form from the other in the same ways that you enter a form from your original shape. If you reach zero Hit Points in either, you revert to your humanoid form.
When you reach 14th level, your chosen shapes begin to affect you even when they are not in use. You may perform any action or ability that your chosen forms have access to, so long as that form has not been reduced to zero hit points, even if that ability is clearly impossible because you are not in that shape. If you use an ability that requires you to have tentacles for example, your hand actually temporarily transforms into tentacles (in a most disturbing fashion). If you need to maintain that state longer than your turn (for example, if you have grappled somebody with your tentacles, and don't want to let them go immediately), you may do so.
You may perform or maintain a number of actions in this way equal to your Constitution Modifier. Maintaining such a change for longer than your turn is considered an additional use of the ability; the change can then be held until the end of your next turn, and potentially maintained again then. You regain all uses of this ability during a short or long rest.
Worth noting: this usually looks quite horrifying, even when it might be expected to be "cute". Any use of an ability from another form will result in some obvious physical change, even if it's something that could feasibly be done in human shape (like a bull rush from a Bull).