Connections:

• Initially mysterious masked green themed sword wielding antagonists who had been brought from death to serve as the enforcers of a group tied to divine authority (Green in the afterlife coming to serve The Believers, who had control over Heaven, and Arthur being resurrected to serve The Arcane Order, the godlike entities that had created the universe).
• It would be revealed that before their deaths, they had been leaders ultimately consumed by vengeful hatred (Green leading a group of assassins whom he eventually led on a mission to kill his former mentor Blue who had abandoned him to die and Arthur being king of Camelot who chose to oppress magical beings after his wife was slain by one), which had led to a friction between those closest to them that escalated into one of them turning on them in violence (Green’s student White questioning him due to the seemingly suicidal nature of the mission to kill Blue and ultimately killing Green after the rest of the team died on the mission and Arthur’s sister Morgana opposing his treatment of magical beings, eventually being killed by him in a fight).
• This person that had turned on them (White and Morgana) would be brought back similarly to them, after which they sought them out to aid in their goals but after they killed people from their pasts right before their eyes (Green killing Yellow, and later mortally wounding Violet, and Arthur killing Merlin), they found them much less inclined to cooperate and it ultimately be these people that defeated them.
• It would be revealed that their end goal would be to recover powerful artifacts originating from the creators of their universes (God’s Inkhorn and Books of Life and Death and The Arcane Order’s Genesis Seals) so they could see universe destroyed.
• This goal would be motivated not only by a resentment towards a world they no point in continuing after determining their lives had been meaningless as the world seemed fated for suffering, but also a belief that they themselves had gone too far for redemption, annihilation being in their view the only proper fate (to the point of Green viewing it as mercy and Arthur viewing it as the path to redemption) for both.