This Chapter is for readers who don't know how Ten Shadows works.(Or maybe for everyone as I explain how it translates to One Piece world)
But I have to say, If you haven't read Jujutsu Kaisen, you're missing out.
Ten Shadows allows the user to summon and control up to ten shikigami, which are essentially spirit beasts. These creatures are conjured using the user's own shadow as a medium, and each has a unique role in battle—like offense, defense, crowd control, or even distraction.
The user makes a shadow graphic hand sign to summon these shikigami, and once a tamed shikigami is destroyed in combat, it's permanently gone—but it's traits can be passed on to other summons making them stronger.
When a user first gets this ability, they are given the First set of shikigami, a pair of twin wolves called Divine Dogs(Yes, they count as a single shikigami) to assist the user.
To tame the remaining 9 shikigami, the user has to defeat them in mortal combat without any external help(the user can use other shikigami, just not other people).
If there is any external help during this ritual, the Shikigami will not be tamed even if you kill it.
User can summon an untamed shikigami as many times as he wants in order to try and tame it.
So it's like Pokemon, except with life-or-death stakes and creepy mythological creatures.
Another aspect of this ability is combining tamed Shikigami to create hybrids,there are two ways to do it.
1) First one is called Well's unknown abyss, users can combine any tamed Shikigami to create hybrids and these can be destroyed without any consequence, but these are very weak and can at most be used for auxiliary tasks.
2) Second is called Totality, as I mentioned above, when a tamed Shikigami dies, its traits are passed onto others making them stronger. Hybrids summoned by using Totality are typically stronger than their base from and act like normal Shikigami, that is if they are destroyed they are gone.
One more aspect of the ability is control over shadows to a degree, user can move across shadows, into the Shadows to hide and store items inside their shadow(User has to bear the weight of anything that is stored in their Shadow).
Example of abilities of some Shikigami
Divine Dogs- Strong sense of smell
Nue- Flight, Electricity
Mahoraga- Ability to adapt to any and all phenomenon (it's ridiculous)
In Jujutsu Kaisen, a sorcerer can learn an ability called domain expansion.
Domain expansion of Ten Shadows or in this case Megumi is called Chimera Shadow Garden, where Megumi expands his shadow to cover some amount of area, inside which his control over Shikigami increases considerably.
So how does this translate in One Piece world
In Jujutsu Kaisen all cursed techniques are fueled by Cursed energy, whose capacity is fixed from birth and you can't increase it.
In my story Megumi's technique still functions, but it's now more like a Devil Fruit power with a stamina-based limit.
He can still summon shikigami like the Divine Dogs, Toad, and others, but it drains his stamina instead of cursed energy.
Summoning too many or using them for too long can leave him physically exhausted.
But the Shikigami can also grow stronger with him and aren't capped by Cursed energy like in Jujutsu Kaisen.
So…that's it. I think I didn't miss anything.