We know that Brightlord Dalinar Kholin was cursed to forget all memories of his deceased wife when he asked the Nightwatcher for a favor.
Was it ever mentioned or hinted in the Stormlight Archive series what wish the Nightwatcher granted for Dalinar? I can't recall any details myself, but I wonder if I just missed it.
At least as of the point where you asked this question, no, you haven't missed anything. Neither Way of Kings nor Words of Radience make any mention of what boon Dalinar received in exchange for the Old Magic. Dalinar's inner monologue makes it clear that he remembers exactly what his boon and his curse were, but he doesn't want to discuss it with his sons, and seemingly reluctant to even think about it to himself.
The fact that Sanderson has made such an issue of Dalinar's curse but never managed to explain his boon makes me suspect that it's going to play an important role in the plot of a future novel.
There is a popular theory that the loss of his wife's memory is actually his boon, and that we just don't know what his curse was. Personally, I think the way he describes his memory loss (using phrases like "taken from him") suggests that it was not voluntary, but it's not very decisive either way.
I know this is an old post, but this has recently been confirmed in Oathbringer.
Dalinar went to visit the Nightwatcher to forget his pain from accidentally murdering his wife at The Rift. Dalinar went into a downward spiral after that event. He becomes "haunted" by the memory of her and all of the innocent he killed that day.
Oathbringer Page 824 Chapter 86: That Others May Stand\
After the memories of the Rift returned, Dalinar finally realizes why he can't remember his wife's name.
The coward's route he'd obviously taken- though he could not yet remember his visit to the Nightwatcher, he knew what he'd asked for. Relief from this awful burden. The ability to lie, to pretend he had not done such horrible things.
Oathbringer Page 1077 Chapter 114: The Cost
Dalinar Actually meeting the Nightwatcher and Cultivation herself.
Interaction w/ Nightwatcher:
What would you like? she asked. *Renown? Wealth? Skill? Would you like to be able to swing a sword and never tire?
"No," Dalinar whispered.
*Beauty? Followers? I can feed your dreams, make you glorious. Her dark mists wrapped around him. The tiny tendrils tickled his skin. She brought her face right up to his again. What is your boon?
Dalinar blinked tears, listening to the sounds of the children dying in the distance, and whispered a single word.
"Forgiveness."
At this point, Dalinar asking for forgiveness confuses the Nightwatcher. Cultivation herself takes things into her own hands.
Interaction w/ Cultivation
After Cultivation asks Dalinar what he wants:
"I asked the ardents," Dalinar said. "I didn't get what I wanted."
YOU GOT WHAT YOU DESERVED. THE TRUTH YOU HAVE CRAFTED FOR YOURSELVES.
"I am doomed then," Dalinar whispered, stopping in place. He could still hear those voices. "They weep, Mother."
She looked back at him.
"I hear them when I close my eyes. All Around me, begging me to save them. They're driving me mad.
THIS WILL BE YOUR BOON. I WILL NOT MAKE OF YOU THE MAN YOU CAN BECOME. I WILL NOT GIVE YOU THE APTITUDE, OR THE STRENGTH, NOR WILL I TAKE FROM YOU YOUR COMPULSIONS. BUT I WILL GIVE YOU... A PRUNING. A CAREFUL EXCISION TO LET YOU GROW. THE COST WILL BE HIGH.
"Please," Dalinar said. "Anything."
IN DOING THIS, I PROVIDE FOR HIM A WEAPON. DANGEROUS, VERY DANGEROUS, YET, ALL THINGS MUST BE CULTIVATED. WHAT I TAKE FROM YOU WILL GROW BACK EVENTUALLY. THIS IS PART OF THE COST. IT WILL DO ME WELL TO HAVE A PART OF YOU, EVEN IF YOU ULTIMATELY BECOME HIS. YOU WERE ALWAYS BOUND TO COME TO ME. I CONTROL ALL THINGS THAT CAN BE GROWN, NURTURED.
A BOON AND A CURSE the, Mother said. THAT IS HOW IT IS DONE. I WILL TAKE THESE THINGS FROM YOUR MIND. AND WITH THEM, I TAKE HER.
"You'll take..." He spoke with difficulty. "You'll take Evi from me?"
ALL MEMORIES OF HER. THIS IS THE COST. SHOULD I FORBEAR?
Dalinar squeezed his eyes shut. Evi...
He had never deserved her.
"Do it," he whispered.
Though Cultivation didn't give Dalinar the usual Nightwatcher boon/curse combo, she gave him a "pruning". It is argued that Dalinar didn't receive a boon/curse combo, but I believe he did. Cultivation took all of the memories causing Dalinar guilt (the boon). With those memories, comes all the memories of Evi (the curse).
