this morning's not-so-happy thought is apparently regclar
Knowing that using magic drains the king, Clarus volunteered to take on some of the burden from Regis.
Regis always had a way to let people share his life force, but he never did it. Until Clarus asked.
Traditionally, the Lucian monarch's family (especially the spouse) shares their life force, so the burden of dispensing magic is spread out. But people without Lucian royal blood die faster, unless they're really, really strong of body and heart.
Many Lucian monarchs end up NOT sharing their life force with others after their loved ones die, so they soon follow. Romantic texts sometimes say the monarchs died of a broken heart. And that may well be the truth.
Aulea had wanted to share Regis' burden, but he had refused. She died of natural causes soon after Noct's birth. This cemented Regis' decision to never share the burden with anyone, especially not his motherless son. And especially not after he learned about Noct's fate.
Noct would have a good life, while he could. Noct would not share his burden. Even if it means Regis dies early, and Noct is left an orphan. At the very least, Noct would have some good years.
Then Clarus stepped in. At first he used Noct as an excuse...
"He already lost his mother. Do you want him to lose his father so soon, too? Don't you want to spend more time with him?"
But the both of them know Noct is just an excuse - Clarus would die for Regis. They knew this long ago. If it's in his defense, or sharing his burden, or both, Clarus would do it.
So Regis let him share the burden. Clarus was strong, so he was able to keep up the charade of not weakening for a long time.
The people saw Regis needing a cane to walk...but the fact was, Clarus was also struggling. His children might have known, but no one else did. And his children did not know why.
In fact, NO ONE knew that the King and his right hand had made a pact: they would be together until the end, no matter what form the end took.
Neither of them was going to let the other die of heartbreak.