That Time I Got Reincarnated as a Slime: Trinity in Tempest (Manga) 6

The new spinoff manga to the hit fantasy isekai series, developed by original creator Fuse! Against the odds, the little slime Rimiru has established his magical kingdom for all monsters, called Tempest, and it's thriving. But three visitors, Phos the fox girl, Stella the dragon girl, and Nemu the winged girl, come to pay Tempest a visit, they're stunned at just how quickly it's developed. Don't miss this new story in the world of Slime, just as the second season of the anime arrives!

THE DEMON LORD RISES!
 
So many monsters have lost their lives at the hands of the Falmuth army! Phos and the rest of the Trinity feel guilty for failing to protect them. Lord Rimuru, bereft because of Shion's death, secludes himself for four days, then makes a momentous announcement. "I shall become Demon Lord!"