SWORD OF THE COSMIC GODS
According to the Lovecraftian Mythology, Cthulhu and Hastur are half-brothers, but they consider each other as arch-nemesis. They are so like Rikkun and Diadem. They are brothers-in-law (After Rikkun and I get married... I'm so excited!) and consider each other as arch-nemesis. Diadem is responsible for killing three of Rikkun's friends so Rikkun has a valid reason.
I don't know about Diadem's side though.
Rikkun is able to control his powers as Cthulhu without his heart being cursed. That's generally a good thing, but the other threat remains. Diadem is going all out, and he's determined to kill my lover.
"I have chopped your head for about a million timelines. It would be satisfying if I can do it again, Alaric Eisenhower!" Diadem/Hastur proudly said as he jumped forward.