Onigashima and the Skull Dome are essentially a metaphor for Kaido’s energy, and Onigashima takes damage as Kaido takes damage. Part of it is directly confirmed, with the island crumbling over time as Kaido’s flame clouds weaken, but I thought I might look more into this. Off the top of my head, these are all the times Onigashima gets majorly damaged in some way:
– Zoro cuts off one of the skull’s horns. I guess under this theory, the fight with the Supernovas was when Kaido actually started to first show signs of minor damage.
– Big Mom blasts a massive hole into the side of Onigashima with Hera. Just before that Zoro scars Kaido with Ashura and Luffy starts punching him with Conqueror’s Haki.
– Momonosuke blindly crashes through Onigashima and knocks several holes into it, most notably a hole just above its eye that kinda looks like a scar. Since this was just when Yamato’s fight with Kaido was wrapping up, I guess this represents the damage she did to him.
– Luffy vs Kaido rematch starts, and the island starts crumbling from the Flame Clouds weakening. Pretty much explicitly stated by the text, as I said. Hell, its the first actual sign given that these fights are wearing Kaido down.
– Law pierces through Onigashima with Puncture Wille, and Big Mom accidentally detonates a bomb that shakes the island and creates a big hole in the basement. This is the most spurious part of my theory, as I’m essentially saying this represents all the off-screen damage Luffy did to Kaido before entering Gear 4th.
– Kaido pierces through the island with his fire breath while fighting Gear 4th Luffy. Represents the damage from Gear 4th, obviously.
I’m not sure whether this counts as damage or not, but during Kaido’s fight with Gear 5th Luffy the flames of the burning castle become so intense that vast amounts of smoke pours out of the skull. You can argue this represents Gear 5th damage if you want, maybe not.
In the latest chapter, Kaido melts the other horn of Onigashima before clashing with Luffy. Although he’s using what looks like some super ultimate technique, ultimately he’s at the last of his rope and about to be defeated.
You can probably punch a lot of holes in this, and I wouldn’t be surprised if Onigashima takes no damage after Kaido gets knocked out (though I wouldn’t be surprised if something happens like Momonosuke bungling up landing the thing and essentially just doing a more controlled crash), but whatever. It’s a fun thought exercise for me.
Theory by Dreadnautilus (https://www.reddit.com/user/Dreadnautilus)