(This is not my idea, I saw it it the comments and it was so deep)  

3rd pov

Nightmare came to his base crying, when the team tried to calm him down he told them the whole story. They were very worried,  because Dream died a long time ago. Positive sibling's death was some time after signing up the peace treaty between Bad and Star sanses.

He died from the wounds he got while trying to convince the enemy team to stop fighting, he did it, but later on he left this world in his own room with Nightmare holding his hand saying that he's sorry.

Everyone realized how Nightmare needed professional help, and was hallucinating, maybe from sadness, maybe it turned him slowly even more insane, or even it could be the guilt.

-We will go get help Nightmare - Killer said, and hentai tentacle jumped up to him.
-Really?! How do we do that?! Let's go before it will be too late!

Killer grabbed his arm and opened a portal, he knew that Nightmare will quickly realize that they are not going to help already dead Dream, but they plan to get him a therapist, or maybe to lock him in a mental hospital.

They were inside of the building now, and Killer walked to Sci while someone from the team was still next to Nightmare while the other skeleton explained everything that happened to the doctor.

-Okay, I understand, Nightmare can go with me, we need to help Dream as quickly as possible, but you have to firstly come to answer some questions in my office okay?  - Sci knew that he should keep the cover because Nightmare won't let himself be locked in there while he thinks his brother is in danger.

Tentacle man followed Sci to his smaller room and Sci began to question him different things, he used different excuses for it when Nightmare was getting suspicious, Nightmare did have hallucinations even meanwhile tests.

Sci couldn't look at it anymore, he knew everything already anyways.
-Okay, I know what we should do...
Nightmare, you're hallucinating all this time, you need medical help, we will rescue Dream if you will agree to seek help for your own self okay? - Science blackmailed him and Night just nooded in agreement.

They got Nightmare assigned to the suitable segment of the mental hospital and planned how to cure his mental illness, they knew that there surely was depression after loosing Dream, but they also quickly realized that surely there was also something more that was eating him from the inside and was making depression more active and destructive.

(I wanted to post it today but I have to go attend my classes, I will finish it when I'll be able to, thank you for all the support and the fact anybody actually reads this)