I have really enjoyed all the lore bits so far. The new two bosses are very fun to fight against. And it's really nice that Rouxl Kaard got some fun dialogues, as well. Overall, a very fun adventure, albeit with some tough and tricky enemies on later levels, which is a very fine experience. With that said, I have some problems with it that should be addressed.
GRAMMAR STUFF (and some lore stuff)
A LORE MISTAKE WAS MADE
I think you made a mistake. The Kindness disk is used TWO times in the story, even though the friendship disk is never used in the adventure at all. I recommend changing one of these to "Friendship Disk", so your lore doesn't contradict itself, since you said that Luigi only has one chance to use each one of the disks.
GRAMMAR STUFF THAT NEEDS TO BE FIXED
It should be "await" instead of "awaits".
It should be "Prepare" instead of "Prpare"
It should be "tricks up my sleeve" instead of "tricks in my sleeve".
It should be "when we meet again", instead of "when we will meet again"
It should be "what's funnier" instead of "more funnier".
Also, I suggest changing "This place will be your grave" to "This place shall be your grave". It sounds more natural that way.
It should be "I was waiting", instead of "I'm waiting". Since Numbuh One was already expecting Luigi's arrival.
It should be "travelling" instead of "traveling". Consonant, vowel, consonant rule.
It should be "sacrificial" instead of "sacrifical".
How are you is a question, so you should replace the "!" with a "?".
It should be"I obtained Derik's power" instead of "I obtained the Derik's power".
It should be "I transformed into Derik", instead of "I was transformed into Derik".
It should be "It seems that Derik's power" instead of "It seems that the Derik's power".
It would sound better if you wrote "I have no choice, but to become a vessel to this new body", instead of "I have no choice, but to become a vessel to that body".
It should be "to seal Derik's power" instead of "to seal the Derik's power".
It should be "he will", instead of "he would".
It should be "you had fun", instead of "you have fun".
It should be "travelled" instead of "traveled", for the same reason I mentioned above about the consonant, vowel, consonant rule.
Instead of "for retrieve", it should be "to retrieve"
It should be "Luigi only has", instead of "Luigi only have".
Once again, it should be "travelling" instead of "traveling".
This second paragraph could be written like "The Delightful Children were originally the Sector Z from Kids Next Door before becoming evil because of Father's influence". It sounds more natural.
It should be "travelling" instead of "traveling".
LEVEL DESIGN RELATED STUFF
FINAL LEVEL
This pipe leads to the level 00, make sure to use the correct pipe type. Oversights like that can make the player lose progress.
SWITCH DOORS ARE NOT WORKING PROPERLY
While the Yellow and Green Switch doors work as intended, the same can't be said about the red and blue ones. Completing the red switch doesn't unlock ANY door at all, while completing the blue one unlocks both the red and blue switch doors. Make sure to check your blocks and etc and solve that as soon as possible.
WEIRD OAM ISSUES WITH A SPECIFIC BOSS
I replayed the semi-final level twice and it's weird. This boss sometimes show some glitched sprite graphics, sometimes not. While you can just delete these graphic tiles to mask this issue, I would recommend talk to someone who understands of ASM and try to find a solution for this weird graphical glitch. It also happened mid battle when everything seemed alright. You must fix this sprite as soon as possible.
-- x --
Now, for a small suggestion:
This submap looks quite empty. You could add some funny memes in the middle of the map. Or a cool character. Anything to symbolize that this is a special world. Just a suggestion, though, if you don't want to do that, you don't need to.