The Neverwinter site has been updated with a new developer blog that showcases some big changes coming to the Boon System. Most notably, the team recognized the need for a unified UI to show all Boons on a single interface. In addition, Boons have now been grouped into five Tiers, with each tier offering more options "allowing you to invest points into the boons you feel most benefit your playstyle or build".
"Above Tier 5 is a section labeled Master Boons":
These represent boons that do a bit more than just give passive bonuses. These boons are similar to the boons you would find on the right or at the top of our current boon windows. Master boons are limited to one. Which means that you can invest points in one at a time. Each rank increases the strength of the boon and may add some additional functionality.
So how do you unlock master boons? Like the rest of the tree by simply spending points. But unlike the other tiers master boons do not cost a boon point. They are a bonus choice you get for investing in the rest of the tree.
Get a good look at the new Boon UI and system by visiting the Neverwinter site.