The humble player steps solemly into the final boss's room only to see...

[Yosher]

https://www.youtube.com/watch?v=nvsE9vOPioc

[Ad Bot]

[Planr]

"wrong forum board" by OP.

[Insert Name Here²]

007 Sound System "With A Spirit"

[Stick Man]

https://soundcloud.com/spor/the-hole-where-your-house-was

[startacker]

https://www.youtube.com/watch?v=7EXBmsFHD0M
or
https://www.youtube.com/watch?v=yeRM5Xv78iI
or just anything from starforgeter/STRFKR

[Space1255]

http://youtu.be/DgfWEjpEhsU

[kongo]

https://www.youtube.com/watch?v=aHCnOafxfis

[Superfun909]

Omega flowey's theme / Your best nightmare

[Shizza04]

https://www.youtube.com/watch?v=XPoiPZYbtnA

[Rigel]

THE CLOWN HAS BEEN ENGAGED

[bigcapitalbaka]

https://www.youtube.com/watch?v=ZY3ufkZPm1s
'ominous'