How NOT to Summon a Demon Lord, Light Novel Vol. 8

Having finally become king of the elves, Diablo and Shera spend their first night as king and queen together... After, Diablo and his group return to Faltra. He visits Edelgard, deals with a crime syndicate...and bathes Klem?!
After this brief pause in their journey, the group makes their way to the northern mountains located in the Demon Lord's domain. Realizing he might not win the upcoming battle with the Demon Overlord, Diablo decides to rely on the swordmaster to help him level up quickly.
"Instruct me in swordplay, Swordmaster! I'll allow it!"
"No one's been so overbearing before..."
Will Diablo manage to master the blade, all for the sake of defeating the Demon Overlord?!
This is the eighth volume of the adventures of an earth-shakingly powerful Demon Lord (or at least someone who acts like one) taking on another world!