Okay so I've seen some speculation on the ending of Dragon's Dogma and what it exactly entails so I thought I'd take my own swing at it. So, here it goes.
When you defeat the Dragon, the Dragonforged, and the Duke are affected, this is because neither of them defeated the Dragon, their hearts are still held within the Dragon, so when you as an Arisen defeat the Dragon and claim back your heart, that also returns the hearts of both the Dragonforged and the Duke to them.
Now the Duke is by far the easiest to explain. He reached the Dragon as you did, but when asked would he stand and fight or leave his loved one behind and gain influence in the world, he chose to sacrifice his wife Lenora (She's only mentioned once or twice I believe, but not really explained in detail). So the Duke took the Dragons bargin. The Dragon would depart for a time and allow the Duke to hold power over the land he was born into. Oh and FYI, the Duke is the ruler of Gransys, the country the games based in), because Duke can be put in charge of countries controlled by a kingdom, and rule over it in the true monarchs stead. In this case the Dukes the true ruler, as far as we're aware, as an actual ruler from another land would be mentioned if it was ruled over by another nation. So basicly the Duke gave up his lover (which you can do for a little cutscene showing you as the new ruler of Gransys, but its not the true ending so you'll be forced to "Retry").
Now the Dragonforged is more complicated, he didn't get his heart back from the Dragon, but his pawn survived (the Fool is his pawn), weather he actually survived or not isn't very clear, though it seems he didn't and because of that, the "immortality" the Dragon gave him trapped his soul on the earthly plain. So I believe he's a ghost, that is bound to the world because he failed to defeat the Dragon. He wasn't ready basicly, which was what the Dragon waits for. The Arisen to get ready to defeat him. He was mostlikely arrogant and cocky when he set out on his quest and thus grew too bold and took on the Dragon much sooner than you did. So he ordered his pawn to leave items that would lead any Arisen wise enough, to him so he could council them so they wouldn't make the same mistake. Hence why your told to go and visit him before you face the Dragon.
So basicly the Dragonforged failed against the Dragon (the only one I assume who can truly kill the Arisen), and never reclaimed his heart. This was probably quite soon after Savan had defeated the Dragon, and the Dragonforged probably didn't know about him or didn't think much of him. Now the Duke aged, it had only been 50 years remember, and one of the villagers in Cassardis (Iola, the elderly shop keeper) states that he and she are about the same age. The Duke hasn't really been in that much danger since he took the Dragon's bargin so its not surprizing he would have lived and still been able to weild a sword, too some degree.
Now the biggest conundrum people have faced is, Selene. Selene is a pawn that was given humanity. This is actually either very simple or very difficult. Either she was given humanity the same way your is after the true ending, ehen you stab yourself with the Godsblade, and she couldn't handle the shock of losing her Arisen (now that she has human emotions she'd react like we would if we lost someone we loved very dearly, some simply fall into depression and this would explain why she forgot everything until she met you). This seems to have some gaps though, such as what about her Grandmothers spirit (her Arisen). Well she might of grown to care for Selene so much she held on to the world, and stays there still to tell other Arisen about this connection and gift they can give to their pawns if they truly care for them. But this still seems flawed to me so lets try a different approach.
When her Arisen faced the Dragon, she asked that her heart be given to Selene instead, and that she lose her immortality (because the Dragon is based around Angels believe it or not, and has connections to the Senechal, who in their own way is a God). She may even of just asked for her pawn to gain a heart or to share her heart with her pawn, which would explain why she aged and died, but Selene still lives, because she got the heart after her Grandmother died, and it would take time for it to meld with her, and it would take even longerwhen she wasn't in contact with other humans for her to learn from, as having a heart was new to her. But some might not agree with this, as it still has gaps.
Really it might just be that Arisen have affects on their Pawns, without even realizing it. So Pawns can become human, if they spent more time with a human. The problem is they begin to act like THAT human, and not as an individual. Basicly Selene is where all the real mysteries show up.
But concerning your Arisen and Main Pawn at the very end of the game? You sacrifice yourself in an attempt to break the cycle, to break the chain, and knowingly or not give your body and heart to your Pawn. Personally I like to think that your Arisen used their powers as Senechal to ensure that before their lights went perminantly out. But that's just me, as I love happy endings.
So that's it, I gave it my best shot and I'd love to know what you think. Do you have any theories abou the ending?