Hook for the Heroism Paladin: he's an incarnation of a sort of Jungian / Campbellian Universal Hero. The longer he adventures, the less he remembers about the personal history of this particular incarnation, and the more he settles into the role of Hero and remembers tales of yore as his own life. This life is the latest telling of an eternal story.
The rest of the party lives in the comparatively mundane medium-fantasy world, where you might know some magic or fight a dragon or whatever; this guy is genuinely convinced that his father was The Sky, his mother was The Earth, that he beat Old Age in a wrestling match and beat Fire in an eating contest . . . and the longer you hang out with him, the more you believe it.
I basically played next to this in a campaign that ended a few months back. The charismatic barbarian was completely convinced he was the king of this ancient land, Pal, that no one had ever heard of. The player was knew he was delusional.
Anyway, the rest of the party went along with him. Some true believers, some humoring him because he was a remarkably effective leader. He gave us all positions in his court, my fearless halfling bard was The First Herald of Pal.
And then as we were negotiating with a dragon, the DM had the dragon throw in that what was once the Kingdom of Pal was far to the north, near the arctic circle. Suddenly all of the doubters were doubting their doubts. And the player of the barbarian was like "Wow, that's cool!".
So now we had to figure out if it was true. We ended up getting there, finding that after being involved in the Dragon Wars thousands of years ago (which was a well established part of the campaign and was looking to reoccur), the King of Pal had disappeared, vowing to return when his people once against needed help against the dragons. At which point the people of Pal dispersed. And we found the legendary Hammer of Pal, which much like Mjolnir could only be wielded by the true King of Pal - and did not let the character wield it.
So we were still uncertain if we were with the true king of Pal, or just someone with delusions.
Long story short, we did some really epic things, died together and escaped the underworld, became members of a tribe of barbarians that were descended from the people of Pal and were able to convince
all the tribes to come back. Then the Hammer accepted him. We returned to Pal finding several dragons and an army of their servitors trying to take Pal, fighting the barbarian families that were returning. In an Epic battle we fought and slayed them, and once again the Throne of Pal was occupied by the True King of Pal.