Just look at the OC comments in Fallen_So_Far  Undertale Roleplay if you don't know who Ausia/Nightmare is :3

It's a beautiful day outside
Birds are singing, flowers are blooming
The guy who said that?
He no longer exists~

Who am I now?
Who really knows
But I'll show you that I'm no longer who I used to be
Now many fear me

Probably cause I took over all timelines

Everyone is simply a fool
For thinking that I have the capacity to not be cruel
Let's go as the whole world crumbles
Let's go, I'll kill the killers

Go ahead, try to hit me if you're able
Now you know I'm not really stable
You think I'm simply a new demon?
Because of that the timeline will worsen

Why not just give up?
This is all a game!
Not every player is the same
Some write some very odd things about you now
So much has come from your existence

I'm worse than, L. O. V. E.
Oh~!
L. O. V. E.

L. O. V. E.
Oh~!
L. O. V. E.

After I've erased
You from everything
No one will remember you, even those humans
Even if they try to, the act is useless
You are gone forever now
Because you do not exist!

Go ahead, try to hit me you're able
Now you know I'm not really stable
You think reality will exist forever?
When I'm done it won't be found whatsoever

I can make you just disappear
DETERMINATION has no effect on me
Fight me I DARE YOU
You really have NO CLUE
How far my power extends...

I'm worse than, L. O. V. E.
Oh~!
L. O. V. E.

Of course I'm stronger than you

L. O. V. E.
Oh~!
L. O. V. E.

Settle the score is what I'll do!

L. O. V. E.

I am a Nightmare.