Death, is nothing more than an end to one adventure but it doesn't mean that you won't have another adventure. Because death itself is an adventure all on it's own, but if a soul isn't ready for that adventure it ends up being reborn many years later. Sometimes death let's them be reborn if they spent to much time in deaths realm, this goes for monsters as well.
For one special person this is true, it has been a thousand years since their death and it has been to long for this person to stay any longer in deaths heavenly realm. It was time for it to be reborn but with a different destiny, death knew with this new destiny this person won't be coming back for it will be tied to another which come before the persons two thousand years ago had come to him and with these two together they would be unstoppable. They were destinied to be twins in this new life, he smiled as he released these two back to the living.
Now he wondered what was going now that he wasn't able to watch their lives anymore, he already knew that what ever happens he knew that the monsters still living will be in for a surprise.
I'm new to Undertale stuff so please don't be too hard on me, if you have any questions I will answer them. But I won't give out the full story because I rather keep you guys guessing. *gives readers cookies* See you later!