That Time I Got Reincarnated as a Slime volume 18

DANCING ON PUPPET STRINGS
 
At the Walpurgis, a gathering of those at the pinnacle of monsterkind, Clayman finally plays his hand. The master of marionettes accuses Rimuru of attempting to usurp his Demon Lord status, and employs every trick in the book as the battle rages on multiple fronts. But if there’s one thing this confrontation makes clear, it’s that in the world of Demon Lords, only the truly mighty can stand tall...