Thanks for the chapter!
What MC means about having no choice but to attend basically means the invitation isn't a request but an order from the king to appear or else. Saying it's an invitation is just polite formality to make the king look benevolent rather than domineering to others. Those in the know, like nobility, will realize that if reject the invitation, especially after hearing the king is attending as well, will essentially be insulting the king by implying they don't need to obey him or he's not worth them meeting and thus earn the king's anger. Said anger can be expressed as sending soldiers over to arrest for disobeying the king, harassment by any and all government run organizations such as businesses, gate guards, etc. exile and imprisonment. It's considered understood that the invitation is a direct order and attendance mandatory.
In case anyone not familiar with monster rooms (or more commonly known as monster houses,) it's basically a trap room that's do or die for parties. Usually what happens is after last member of a party enters the room the door will shut and seal (how it knows when last member enters never explained, or if door doesn't shut a one-way powerful barrier will allow more people to enter but prevent leaving) or somewhere else in the dungeon an activated teleportation trap warps anyone within range of it or the whole party to the monster house. Within the room there'll be a large number of monsters, usually of various types, sometimes stronger versions than same type wandering around loose in dungeon, and only way to leave the room is killing every monster in the room. Usually drops are more numerous and better quality and in some cases a chest will be present or appear after last monster dies. Although if dungeon designer a warped individual with a severe sadistic streak that final chest (assuming it's not a mimic, [monster pretending to be a chest so can ambush adventurers who've lowered their guard,]) will have a real nasty trap or contents in addition to good treasure. Like if don't disarm trap before opening chest an instakill trap will be triggered, such as releasing poisonous gas that fills the whole room or releasing a swarm of ultra high level monsters (like undead spirits, wraiths, reapers, etc..) while locking the group inside the room again. For example, a level 2 dungeon floor intended for adventurers around levels 2-5 has a monster house cleared but the group being too excited over the chest just open it without having a thief examine it trigger a trap that releases dozens of level 100 undead reapers (basically skeletons in robes wielding scythes like a pale imitation of Death that are part physical and part spectral [like ghosts, spirits, apparitions, etc.])
