Boss Battle Fusion Collab

The "Boss Battle Fusion Collab", initially titled "??? Fusion Collab" before being renamed, is a high quality collaborative medley arrangement of various boss battle themes. It was initially premiered in the stream "Join us for the reveal of a mystery Fusion Collab!".

Jokes
This collab has a unique structure from SiIvaGunner's previous fusion collabs; instead of being centered around a single song, each segment aside from the "Megalovania" opening is an arrangement of a boss battle theme (or the song associated with the boss in the case of Wood Man, or an equivalent thereof in the case of LazyTown and Yellow Submarine) from the game(/show/movie) of the previous segment's style. At 6:08, the "One Hell of a Time" arrangement is abruptly cut off by the transition used for King for Another Day Tournament reveals. We then see the various visuals for the fusion collab's segments drift by, set to a calm, jazzy medley of the various themes based on "It's Raining Somewhere Else" from Undertale, ending with a "Boss Battle Fusion Collab" title in the Undertale logo's typeface.

After the screen fades to black, we suddenly get a "reveal" for Sans as a contestant in the King for Another Day Tournament. However, the logo's text turns into:

SiIvaGunner lol nahhh cmon dude sans literally already said he didnt want to be in the Tournament

This text references the reveal trailer "Knock Knock Knock!".

Instead of "SUMMER 2019", the text changes to "GET DUNKED ON", accompanied by Sans' soundbite.

Trivia

 * The other fighters shown in the roster during the "Chaos King" arrangement reference SiIvaGunner's other original fusion/medley collabs:
 * Steven: We Are The Crystal Gems Fusion Collab
 * Wood Man: Wood Man Fusion Collab
 * Kass: Kass' Theme Fusion Collab
 * Adam Levine: Soap Disco Fusion Collab
 * Grand Dad: 7 Grand Dad Fusion Collab
 * Quote: Balcony Fusion Collab
 * Galeem: Lifelight Fusion Collab
 * Snowman: Snowman Fusion Collab
 * Altair: God of ink fusion collab
 * Log: Yule Log Medley Collab
 * Mario: Super Mario Land 2 Fusion Collab
 * The placement of the "fighters" on the grid also references the release date of their respective Fusion Collab in a similar fashion as in the original Super Smash Bros. character select screen, with the 8 characters in the center representing the oldest collabs chronologically and the 4 on the outside representing the most recent ones.
 * The "Chaos King" arrangement was later made into a full rip, titled "Chaos King - Deltarune".
 * By September 21, 2019, the video had already become the most popular rip of Season 4. It is the only video from Season 4, the 3rd video outside of Season 1, and the 21st video overall to reach one million YouTube views; this was achieved on June 7, 2020.
 * The album version stops at the end of the "It's Raining Somewhere Else" medley, before the fake Sans "reveal".
 * Coincidentally, the segment in-between Super Smash Bros. and Cuphead predicted the inclusion of Cuphead as a Mii Costume for Super Smash Bros. Ultimate more than four months after the rip was uploaded.
 * As of 22 January 2022, this is the most viewed rip to acknowledge either of the King for a Day tournaments.