I told you guys the Blood soaked serpent was the red line. It's also MOSA-kun, was controlled by a previous queen of the Gorgon bloodline into fighting, and Blackbeard's objective is to unsealed it with Hancock's fruit and use her lineage to control it. It's what Loki was talking about when he said if he was there he would have killed everyone.

That's why he got Devon to copy Saturn so he could get S-Snake under his control, he no longer needed Boa so he fled with Coby and plotted stealingthe seraphim which he learned of midfight. Mosa-Kun had to go quiet due Blackbeard attacking Marijoise. Loki will fight to free him in the final fight. And when he goes back to hell, the water levels will lower, the all blue will appear, Laboon will be free, and everyone will be free to travel.

>Shanks had Abyss Mark on his arm
>Blackbeard still scarred him anyways and it never regenerated
>BB doesn't have CoC
>BB didn't have the darkness fruit yet
>BB has special lineage to the point even Saturn seemed annoyed about his existence
>BB can eat multiple fruits
>BB was one of the people aside from Luffy that Imu had a torn up poster of
>Luffy referred to BB as "they"

What is Blackbeard? It's clear he's going to usurp the World Government at this point and is the true threat. He has been playing them like a fiddle for years and it's clear is going to steal the seraphim. And he wanted Boa for something, likely tied to the Serpent given she's a Gorgon, perhaps the red line really is the serpent and he's trying to break the stone around it with Whitebeard's fruit and unpetrify it with Boa's then control it with her Gorgon blood.

He's also the depicted Black Nika and likely the destroyer Nika.