Azure Dragoon powers come from being "chosen" by Nidhogg's Eye (the first one that was passed down by Azure Dragoon to Azure Dragoon). This was in Estinien's possession in the DRG quests 30-50 but this eye even chooses you for some reason, granting you powers as well as Azure Dragoon's signature ability Dragonfire Dive.

Then in HW, Estinien absorbs both eyes and gets a red Gae Bolg and enhanced powers. After defeating Nidhogg, the eyes are lost. Even if the eyes are not in physical posession, the Azure Dragoons retain the power bestowed by the eye through an unseen mental link.

That said, after the credits of SB, Estinien finally destroys Nidhogg's eyes, yet his Gae Bolg remains red and, we assume he keeps his powers (as do we).

So what's the fate of Estinien and our powers as Azure Dragoons now that we now longer have a link to Nidhogg? Wasn't that how we were able to summon forth the Dragon Within?