The Ultimate boon
The Ultimate Boon is the specific direct response to the call to adventure.
The Ultimate boon does not necessarily happen at the end of the film, often it happens around the end of the second act. Very clearly Joseph Campbell indicates the Ultimate boon happens before the Magic flight stage and what some people consider the Ultimate Boon stage is actually the Mastery of two worlds stage.
In Star Wars the Call to adventure is the Princess is in peril and needs rescuing “Help me Obi Wan Kenobi, you’re my only hope.” Leia is rescued at the end of the second Act, That’s the Ultimate boon. Luke’s response to the Call to adventure is at the end of Act one when he decides to join Obi Wan, learn the ways of the force and become a Jedi like his father. That happens at the end of Act three when he trusts the Force and makes the shot. That’s master of two worlds
In The Avengers, the Call to adventure is Tony is asked to join the Avengers, and when the Avengers unite at the end of Act two, that’s the Ultimate Boon. However, when Tony guides the missile through the hole and destroys the invasion fleet, that’s not the Ultimate Boon, that’s Master of Two worlds
In Kung Fu Panda, the Call to adventure is when Po is selected to become the Dragon Warrior and learn the secret to unlimited power from the Dragon scroll. He learns this at the end of Act two when he discovers ‘the secret’ is belief. That’s the Ultimate boon. However he still has to fight Tai Lun with this knowledge. When he defeats Tai Lun and the Furious Five bow in respect and call him Master, that’s the Master of Two world’s stage.
Ultimate – The Best achievable or imaginable
Final, last, terminal, utmost, extreme
Boon – A Thing that is helpful or beneficial
Blessing, benefit, favor, God send, gift
If you just look at the word Ultimate – it does mean the end but when you combine it with the word Boon, a thing that is helpful or beneficial or even a God send, it means what is needed to be able to finish the quest.
How to use it in your writing
Usually, not always but usually, The Call to adventure or Inciting Incident happens to the hero, it is not usually an action from the hero. The first turning point, Crossing the first Threshold, Break into Two moment is an active choice made by the hero in response to the Call. Usually, not always.
And the Call to adventure has a resolution at the end of the second Act but the quest is not yet over because obtaining the ultimate boon means you are worthy to complete the journey.
The decision made at the first turning point sets off a chain reaction which is completed at the end of the third act.
Sometimes people set up a Call to adventure where it is completed at the end of the third act and struggle trying to fill act two. If this is the case for you perhaps you can tweak these moments.
If your story is working as it is then don’t force it in just because Joseph Campbell says so but keep in mind this set up is common and works in a number of stories.