Seeing as this had two ending and different paths to chose from why not have the plot line of those so let's go!

Mind you, you got the best ending possible

Day's Path

   You refuse Nightmare's offer but he doesn't give up that easily, after trapping Dream in his prison he continued to pester you, eventually growling bored of your 'game' he traps you with Dream.

   After you both finally escape you go on to help AU's, Dream makes them happy while you silently help, no one (except those you help) know you exist, they see you around Dream but they only think he's being kind to you.

Abandon Nightmare

The lest you could do is help Dream, you chose to help him, enraging Nightmare and forcing you to fight Dust.

Eventually everyone is watched Nightmare and Dream fight, seeing as it was pointless for the rest to be fighting.

Fight Error

You decide to fight Error, a stupid idea not like you care, after a hit or two you hear a soul cracking sound, you whip around to see Nightmare and Dream gripping their chest.
   "You...V I L E•C R E A T U R E."

(Now time for that letter)

   "Well, it's been a while, things are better, Error's...dead that means no one to destroy the AU's, Cross travels with Ink now, he may not have an AU but's he's happy, Horror, Killer and, Dust are like the 'star sanes' but stupider, oh I met swap Ink, he's name's Eraser...interesting guy...oh! And he's in love with Reboot or swap Error, ahhhhhh I ship it so much! Ahem, things are better it'd be a million time better if you both were here...anyways, I'll see you two again...one day."
~To my ray of moon and sun light