Mythic Monsters and Spicing Up Boss Fights

In Mythic Odysseys of Theros, there is a new mechanic, though to my knowledge, it is only implemented with one creature.

The great Kraken Tromokratis is the highest-CR creature in the Theros bestiary. Weighing in at CR 26, it's up there among the most powerful monsters in 5E D&D, I think only surpassed by Eberron's Overlords Sul Khatesh and Rak Tulkesh, and the two CR 30 monsters, the Tarrasque and literal five-headed dragon goddess Tiamat.

Like any boss monster, Tromokratis has legendary actions, helping the creature have a more robust action economy to help face down a party of high-level adventurers. However, it also has Mythic actions, which work differently:

Tromokratis has a "mythic trait," which is that when it drops to 0 hit points, it doesn't actually die or fall unconscious. Instead, its four hearts are exposed through the cracking of its carapace (in Theros, Krakens are sort of crustacean-like). Each of these hearts has the same AC as Tromokratis itself, and 200 hit points a piece (Tromokratis itself has a default of 409). To kill the kraken, you need to damage it enough to expose the hearts and then destroy each of them. Otherwise, Tromokratis just grows back its shell and gets to heal up.

Rather than just tacking on more hit points for the players to deplete, however, what actually happens here is a sort of stage transition.

The Mythic ability, Four Hearts, can almost be thought of as a buff that goes off when the monster hits 0 hit points. This then unlocks new legendary actions that are more powerful, including multiple attacks and an area-effect. This lasts an hour, so if the monster escapes, it needs to use a short rest (or long rest) to get these abilities back (which could open up some tactics for the party).

Balancing fights can be tricky in D&D, especially with a large party that can blast through a monster in just a couple rounds. Most fights in D&D, even the epic ones, tend to last only a couple rounds, so it can be hard to time out things like phase transitions that you find in other games.

I'm very curious to see how this would look in action, but so far I haven't DM'd a campaign that gets up to the levels that can use CR 20+ monsters. My Ravnica game will eventually get there (but I also don't want to rush too much because I'd still like to be able to use monsters that are below like, CR 7.)

Indeed, given that the final leg of the campaign, as I see it, will involve a plane-hopping scavenger hunt for the pieces of a powerful weapon to use against the big bads, I think there's a good chance that Tromokratis might have one of its components inside its body, for when the party gets to Theros.

EDIT:

Duh, I just overlooked that there are three mythic monsters, including a Medusa/Gorgon who sheds her snake skin and a giant spider-lady who gets... covered in tiny spiders.

Effectively, these fights are two fights in a row, so the book recommends giving double the XP for their challenge rating.