(SPOILERS!) Evolution of Secret Bosses in Kirby Games (2008 - 2023) 

game footage by:
Nintendo Unity (Super Star Ultra and Return to Dreamland),
Boss Battle Channel (Triple Deluxe and Planet Robobot),
ProsafiaGaming (Kirby Star Allies),
Zephiel810 & U.D Playthroughs (Kirby and The Forgotten Land) and kirbypwnedyou (Kirby's Return to Dreamland Deluxe)
A compilation of every Secret Boss from every mainline Kirby game. Said bosses can ONLY be fought in the hardest Arena and not anywhere else (Kirby and The Forgotten Land being an exception).
Timestamps:
Kirby Super Star Ultra (Nintendo DS - 2008)
0:04 Marx Soul
Kirby's Return to Dreamland (US)/Kirby's Adventure Wii (EU) (Wii - 2011)
4:35 Galacta Knight
Kirby Triple Deluxe (Nintendo 3DS - 2014)
6:24 Soul of Sectonia
Kirby Planet Robobot (Nintendo 3DS - 2016)
14:26 Star Dream Soul OS
Kirby Star Allies (Nintendo Switch - 2018)
30:02 Void Termina EX & Void Soul
39:17 Retaliator Francisca
40:20 Retaliator Flamberge
41:12 Retaliator Zan Partizanne
42:08 True Void Termina & Astral Birth Void
Kirby and The Forgotten Land (Nintendo Switch - 2022)
52:05 Meta Knight
54:13 Phantom Meta Knight
56:01 Chaos Elfilis
Kirby's Return to Dreamland Deluxe (Nintendo Switch - 2023)
1:04:33 Galacta Knight
1:05:27 Magolor Soul (True Arena ver.)
this video is an updated version of this one: • Evolution of Secret Bo...