This ability comes from long years of training or learning from an old master for a certain amount of time. This feature includes higher CR forms being able to be taken, improved spell casting through wild shape, and some expanded form lists.

 Starting at 2nd level, you can use your Wild Shape to transform into a beast. You ignore the CR Max. However, you must abide by the other limitations there are on the CR table. The CR you can transform into is equal to half your level rounded down.

 At level 6 you are given the choice of taking Fey, Elemental, or Monstrosity (non-humanoid creature for Monstrosity Choice).

 At level 8 you can increase the CR equal to your level.

 At level 14 you take another option from the earlier list or take another type based on your subclass.

 Moon is Aberration

Spores is Undead

Wildfire is Fiend

Land is non legendary dragons and dragon kin

Stars is Celestial

Shepherd is Fey

Dreams is Fey

Other circles will be determined later by updates or you dm for homebrew.

 At level 16, you nearly have mastered your wild form. To determine CR you have to add Level plus your Wisdom. Circle of the Moon druids double their Wisdom score.