"Maybe now you will understand what it truly feels like to be broken." - Nightmare to Dream, before he lost consciousness.
Several days after Ink died, Nightmare returned to the Doodle Sphere with a few of his shadow Papyruses to end the fight against his brother and warn him about his plans to take over what was left of the multiverse.
Since Dream was heavily weakened by the previous fight against Error and his brother, he was promptly defeated, and Nightmare took his feelings and emotions and made himself more powerful through their corruption. This didn't kill Dream, however, and only left him unconscious. When he woke up, he couldn't feel anything and fell into a deep depression, not aware that every good emotion was slowly fading away.
The first monsters Nightmare corrupted were those housed in the Omega Timeline - only CORE!Frisk, the Underswap skeletons and a few out-codes were unharmed. The three, along with Science!Sans, decided to move throughout the multiverse to warn the remaining survivors, and try to save Dream.
However, their attempts proved to be rather unsuccessful as Dream was deemed to be too far gone and lost beyond help; not only that, but every time anyone tried to help him, Nightmare would do anything possible to stop their progress. Since killing his brother would kill him too, though, he resorted to deadly warnings and threats to the others, such as erasing Dream's memories and having his shadow Papyruses torture him until the brink of death. During this, Science and Error were eventually killed by Nightmare, and the Underswap skeletons were temporarily devoid of feelings until Core found a way to help them.
Knowing that Core would still try to help him, Nightmare took Dream to his own dimension, where he was imprisoned and left alone in the dark to fall into insanity. In the end, during a fight as his friends tried to rescue him, he used a jagged bone to end his own life with, subsequently taking his brother's and permanently ending the conflict. The multiverse, however, began to disappear, as neither Ink, Error, Dream or Nightmare were there to keep the balance anymore.
He only has a few bruises and tattered clothes after the short fight, though his cape is heavily torn at the bottom. He still retains wounds from the previous fights against Error and Nightmare: a gash crossing over his midsection, several broken ribs, a few small cuts and bruises around his skull, a horizontal crack from the side of his cheekbone to the bottom of his left eye and a broken femur.
His pupils are grey/white as opposed to gold. Sometimes, a grey haze will surround him, mostly when he's overly anxious or afraid. Whoever is close to him when it happens will feel drowsy and disheartened.
Once everyone's help began to take effect on him, only his left eye regained its golden color and he recovered enough magic to heal his bruises and, eventually, his broken bones.
He's almost completely emotionless and blank, only having room for sadness, anger and fear. When he's asked for guidance or motivation all he does is discourage people, as he is unable to do anything else for that matter. He is heavily depressed and doesn't like people around him; he will tolerate them as long as they don't get too close or become persistent. He is prone to lashing out.
He's traumatized by the fact that Nightmare often checks up on him to mock and ridicule him, as he has grown severely afraid of him and can no longer care about him as a brother. He's so depressed and discouraged that he never even tries to run away.
Once encouraged enough, though, he partially regained his positivity and was able to care more about others, and felt like he still had something to live for.
He can't make people happy; instead, he makes them doubtful and sad. He is aware of this, and often breaks people's hopes just to make them go away.
He can use the haze that appears around him when he's overly stressed as a weapon, able to shoot it at someone. Though not physically harmful, it causes whoever it hits to lose their motivation for a few minutes.
Even though he can't feel empathy, he remembers what he used to be like and the memories often make him cry and weaken him further, even give him suicidal thoughts. He tries not to listen to encouragements because he feels like he doesn't deserve to be happy anymore.
If Nightmare is close by, he will immediately begin to panic no matter what. Darkness makes him panic as well, and too much of it will drive him crazy.
He loathed Core's company at first and tried to attack them multiple times. Realizing they wouldn't go away, he gave up and let them stay; eventually, he accepted their company and slowly built back their friendship. Deep down, he appreciates their efforts to make him happy, though he can't bring himself to show it.
"Why are you still trying?! Can you not see I'm broken?! Just... why can't you see..." - Dream to Core, one of the first times they tried to help him.
He can't see him as a brother but as a monster, and is heavily afraid of him. Even though he's no longer empathetic, he knows that Nightmare is free to take over the multiverse with him and Ink out of the way and wants to stop him, but he can't bring himself to do anything about it.
"Aww, what is it? Afraid of your big bro?" - Nightmare to Dream, mocking him.
He wasn't at all fond of them and considered them a major nuisance, even trying to attack them once. Papyrus deemed him dangerous, though Blueberry warmed up to him and was the first to truly make him change his mind about their company.
"Why do you still... consider me a friend? I tried to hurt you..." - Dream to Blueberry, seeing as he wouldn't give up.
Even though it's his intention to help Dream heal, he's really untrustful of him, mainly because his paranoia makes him believe he wants to hurt him instead.
- The reason he didn't become like his brother is because he wasn't meant to be completely devoid of positive feelings, so the lack of them "broke" him instead of corrupting him. He is losing his sanity though.
- He's completely colorblind, only able to see in black and white.
- He suffers from slight pantophobia (the fear of everything), haphephobia and post-traumatic stress disorder, and thus he tends to have hallucinations and visions. He's also slightly paranoid.
- Even though his eyes remained colorless, he managed to regain his sight through Blueberry's motivational help, albeit slightly.
- Once able to see colors again, he realized light colors like that of a sunset's soothe him, but not even that ever makes him smile.
- Though he tries to forget everything he has gone through, he constantly remembers what he had done for Ink and how he had failed, and becomes unapproachable.
- He's really straightforward and distant, and will mostly speak in short sentences - sometimes, he will simply remain silent. He'll rarely start a conversation or join an ongoing one, though he may say incoherent things and ignore whoever is with him.
- Due to constant trauma, he often speaks alone and may even drive himself into a panic attack.
- He's insomniac.
- He's aware that Ink's "essence", in a way, aids him by changing the climate of the timeline or guiding him around via pieces of paper blown by a breeze. Core and Papyrus don't believe this and think he's just delusional.
- If he touches any sort of plant or flower, they will wither.