As a Halloween treat, I decided to do my own parody of This is Halloween, but Undertale themed! I initially had wanted to have the lyrics be colored for every character that sings it, but I ran out of time for that. So I decided to put lyrics also in the description to indicate more of what character sings what. :) And there may be a full cast version next year if the rest of the Narrator's musical team wants to do it. :)

Enjoy the parody, and Happy Halloween!

[Frisk] 
Boys and girls of every age 
Wouldn’t you like to see something strange? 

[Flowey]
Climb the mountain and fall down 
Descend to the Underground

[Everyone] 
In the Underground 
In the Underground 

[Toriel] 
Monsters wander, devoid of light

[Everyone] 
In the Underground 
Come and have a look around

[Flowey] 
See us all 
Make the choice to spare, run or fight

[Everyone] 
Now you’re here 
Fallen down 
Deep within the Underground 

[Flowey] 
I am the flower who's your best friend 
Follow my advice or meet your end!

[Toriel] 
Looking for the caretaker? Child, it is I! 
Won't you have some butterscotch-cinnamon pie? 

[Everyone] 
In the Underground 
In the Underground 
Underground 
Underground 
Underground 
Underground! 

[Monster Kid]
In the dark it's our home 
Sing along to the monsters’ song 

[Papyrus]
Down below 
In this world we know 
Everywhere you turn there is a new surprise 

[Sans]
Fun with monster friends
Pun-loving skeletons 

[Asriel] 
But if you aren’t careful it might be 

[Asriel and Flowey] 
Kill or be killed!

[Muffet]
In the Underground 
Dangers at each turn abound

[Alphys] 
Don’t scare them! 

[Muffet] 
It’s all in good fun! 

[Sans] 
Puzzles, jokes and japes

[Napstablook] 
And some Halloween mixtapes
Fill the air with their spooky song

[Alphys and Muffet] 
Don't make a sound 

[Napstablook]
Don't make a sound 

[Sans and Papyrus]
Here within the Underground 

[Undyne] 
I am the one who fills man with fear 
None can escape justice’s spear! 

[Mettaton] 
I am the one built to entertain 
But in combat I surely can reign!

[Frisk, Chara, Human Souls] 
We are the children who fell below 
Surviving through the strength of our souls 

[Undyne, Mettaton, Frisk, Chara, Human Souls]
In the Underground 
In the Underground 
Underground 
Underground 
Underground 
Underground! 

[Monster Kid] 
Underground! 
Underground!

[Alphys] 
From Snowdin to Hotland you go 
As you traverse this world below

[Asriel]
Stranger things 

[Chara]
Will sure be found 

[Asriel and Chara]
In your time here Underground

[Papyrus]
Down below 
In this world we know 
Everywhere you turn there is a new surprise 

[Toriel and Asgore] 
Enemies and friends 
Try to stay determined 

[Asriel] 
In this world of darkness 

[Frisk] 
Magic

[Flowey] 
And LOVE!

In the Underground 
See what will be found!

[Undyne] 
In our capital is where our king resides

[Sans, Papyrus, Undyne, Mettaton, Alphys] 
He is King Asgore
The king that we all adore 
Everyone hail to the mountain king now

[Asgore, Sans, Papyrus, Undyne, Mettaton, Alphys]
In the Underground 
In the Underground 
Underground 
Underground 
Underground 
Underground! 

[Asgore]
In the dark it's our home 
Sing along to the monsters’ song 

[Toriel, Frisk, Chara, Undyne, Alphys, Flowey, Muffet, (Napstablook, Asgore, Mettaton, Monster Kid)] 
La, la, la, la la 
(Underground, Underground)
La, la, la, la la 
(Underground, Underground)
La, la, la, la la la, 
La la la, whee!