In Chapter 925 Moriah went into Blackbeard’s territory to rescue Absolom. After Moriah learnt that Blackbeard had killed him and stole his Devil Fruit, Blackbeard invited Moriah to be apart of his crew.

Why didn’t Blackbeard just kill Moriah?

Does he need Gekko Moriah’s power for something big? We all know Marshall D. Teach is power hungry and will stop at nothing to get it.
• He killed Tatch to steal the Yami Yami No Mi;

• He infiltrated Impel Down after becoming a Warlord to free the Level 6 prisoners so he could recruit them;

• He killed Whitebeard and stole the Gura Gura No Mi;

But this time he doesn’t to get another Devil Fruit or to recruit someone. He needs something else.
Blackbeard is after Kaido’s “Shadow”.

• Moriah can place someone’s shadow inside of someone’s living body, increasing their physical power and possibly granting them a fighting technique.
Kaido is called the “Strongest Creature in the World” and Blackbeard is the “Strongest Man in the World”
Gorosei even said that Kaido is one of the few capable of stopping Blackbeard.
One last power up could put Blackbeard above all the Yonkos.

• Blackbeard craves power and since he’s the strongest man I believe obtaining Kaido’s Shadow the strongest creature is what he is after.

Remember when Moriah used “Shadow’s Asgard”

Moriah absorbed 1000 shadows at once giving him a monstrous increase in both size and strength that allowed him to split his ship in half with one punch.

Moriah VS Kaido – Moriah stole Kaido’s Shadow before!

“Moriah is a pirate that once matched one of the 4 Emperors Kaido”
Kaido was called 100 Beast Kaido. We’ve seen how strong Moriah was after he obtained 1000 human Shadows. It’s possible that Moriah gained Kaido’s Shadow to be able to fight on par with him.
Shouldn’t the sunlight have vaporised Kaido?
Well…..it’s Kaido and he’s unkillable literally. Nothing has been able to kill Kaido so even if gaining Kaido”s Shadow gave him a fighting chance, Kaido could have been able to get it back after defeating Moriah and then killed his crew afterwards for stealing his Shadow in the first place.
• Moriah blames Kaido for killing his crew but he could also blame himself for angering Kaido in the first place which led to his defeat and the death of his crew
Moriah in Wano?
Moriah is currently in a death trap being surrounded by Shiryu “of the Rain” and “Crescent Moon Hunter” Catarina Devon

Moriah is smart and strategic and knows that it’s more wise to accept Blackbeard’s offer then to try to fight them off right now but take revenge for Absalom later.
Moriah could be sent to Wano to get Kaido’s Shadow and it’s possible that Blackbeard himself will go there too.

LUFFY’S NEXT POWER UP – Moriah gives the Shadow to Luffy?
Nightmare Luffy V2 – “100 Beasts” Luffy

Moriah hates Blackbeard more than he hates Luffy right now.
And since Moriah is weakned now he knows that he can’t beat Kaido or Blackbeard even if he were to obtain it for himself.
Fact: Jinbe claimed that Moriah’s dependence on absorbing the shadows of other people has weakened him.
Possiblity: Moriah could end up taking Kaido’s Shadow and giving it to Luffy instead of Blackbeard as revenge for killing Absalom.

Foreshadowing that Luffy will end up with Kaido’s Shadow
• Nightmare Luffy is Luffy with 100 human shadows. Kaido is referred as “100 Beast” Kaido.
Luffy might obtain the shadow of Kaido which is equal to 100 beasts.
• Nightmare Luffy was able to stop Oars “The Devil” who looks similar to Kaido.
It’s likely that gaining Kaido’s Shadow is the Key to Luffy defeating Kaido because
the gap between Kaido and Luffy right now is really big and it will take a major power-up to catch up and fight Kaido equally.

What about Blackbeard?

Ok Blackbeard is not gonna just stay there and let Luffy have Kaido’s shadow.
Blackbeard might end up with another major power-up at the end of the Wano War like in Marineford War because right now Shanks Kaido and Big Mom are the ones who are able to stop him and one more power-up will put him above them and that is what he’s after.
Whether he kills Moriah and takes his Devil Fruit to obtain it from Kaido or takes Big Mom’s Shadow instead.

But Luffy beating Kaido by absorbing Kaido’s Shadow will finally make Blackbeard take Luffy seriously because during the Reverie Blackbeard claimed that Luffy is still not at the Yonko Level.

*Theory by King of beasts kaido