WARNING: The following contains spoilers for Episode 42 of That Time I Got Reincarnated as a Slime, now streaming on Crunchyroll.
Rimuru Tempest finalizes his plans to take over the kingdom of Falmuth and confront Demon Lord Clayman at the upcoming Walpurgis meeting among Demon Lords, but he's far from the only skilled politician in Reincarnated as a Slime. Though he has recruited many allies and successfully spread strategic misinformation to his enemies, one Demon Lord is not so easily duped.
Leon Cromwell was the Demon Lord who originally summoned Shizue Izawa to this isekai world from World War II-era Japan. Now in Episode 42, he meets with Demon Lord Guy Crimson to talk things over and plan his next move. Cromwell is greatly displeased that Rimuru seeks to prevent future isekai summonings, which may very well see them clash soon.
Demon Lord Cromwell arrives at a remote, polar fortress and receives a warm greeting from Guy Crimson, one of the oldest and most powerful Demon Lords of all. They begin a private meeting with Velzard the ice dragon (a sister of Veldora) to discuss all the recent developments in the world. The great Veldora has returned but hasn't yet gone on a rampage. The Demon Lords, including Guy Crimson, believe the false cover story Rimuru spread: the battle with Falmuth forces shed enough blood to awaken Veldora and release him. Rimuru's rapidly developing political skills continue to pay off.
Cromwell and Guy Crimson also discuss the odd partnership between Demon Lords Milim and Clayman, both finding it unlikely that the ancient, prideful Milim would willingly join a "small fry" Lord. However, Cromwell hardly cares about this and has taken a much greater interest in Rimuru Tempest. He recognizes Rimuru is a bigger power player than Clayman at the moment, and is likely to be proven correct in future Reincarnated as a Slime episodes. This bodes ill for the scheming Clayman.
Demon Lord Cromwell has a grand plan to summon people to this world and achieve his great goal, but it remains unclear what that goal is or how it will affect the rest of the world. He also knows someone seeks to prevent all nations from performing the summoning ritual any longer, and strongly suspects it's Rimuru. This puts the two at odds, and the world's other nations may soon get mixed up in their feud. A frustrated Demon Lord like Cromwell is likely to retaliate with extreme force.
Guy Crimson opts to stay neutral in Cromwell's affairs for now, trusting him to handle this problem with Rimuru alone. The upcoming Walpurgis meeting is of far greater interest to Guy -- while he takes a keen interest in the world around him, he's not too quick to meddle or make a rash decision. As Reincarnated as a Slime continues, Guy might serve as an effective mediator and leader when the Walpurgis meeting begins in earnest.