This is Dreammare. I'm not writing them as brothers here, for reasons that might be obvious.
Also, sad warning.

The hanahaki disease had been around for a very long time.

When Dream was younger, and still lived by the village with Nightmare, there used to be a tradition where the one with the disease would write a note to the one they loved, and have a friend or family member deliver it when they died. This was, of course, because they didn't know how to remove the flowers back then. Having the disease meant certain death, unless you could get him or her to love you back.

Since he'd been released from his stone prison, however, he realized that nobody remembered that tradition.

It was rather sad.

Of course, this meant that Ink and Blue would question him when he asked them to take a letter to Nightmare after he died.

First, he had to write it.

He stared at the blank sheet of paper, pen in hand, and tried to think of how to start.

'Dear Nightmare,' he wrote, then stopped.

What did he want to say? That he loved him? That despite everything, he wanted Nightmare to be happy? The last was too cheesy, and the first wasn't enough. Did he want to say that he wished things could've been different? That was obvious already.

After about ten minutes of blank staring, he simply sighed and threw away the paper.

"Hey, guys?" He called.

His legs no longer worked. He was proud that he'd been able to stay on his feet and hide his disease for so long, but all good things came to an end.

Ink and Blue came in quickly. 

"What's up?" Ink said, but his usual cheerful tone couldn't mask the despondent colors and shapes of his eye lights. Even he was upset by his friend's imminent death.

"I-" Dream started, but started coughing. Golden petals were spit into his hand as he tried to cover his mouth.

Blue held him steady while he coughed. Soon, it had passed, and he took a few rattling breaths.

"When...When I die," he started, "I want you to put one of the petals in this envelope." He gave it to Blue. In neat handwriting, he'd written 'Nightmare'. "Give it to Nightmare. He-" he coughed again. "He'll understand what it means."

Ink frowned. "Why would you want us to give it to him?" He asked. "I mean, I guess, he'll be happy to know that you're dead, but-"

"Ink," Blue warned.

Dream cracked a small smile. "Don't worry about the 'why'. You don't even have to give it to him directly; just leave it on his front door. Please?"

Blue hesitated, then nodded. "I don't have to understand," he decided. "It's what you want, so I'm going to do it. We're going to do it," he glanced at Ink, who nodded grimly.

Dream smiled at them. "Thank you. Both of you."

"Nightmare!"

Nightmare sighed. Damn Killer; he just wanted to take a break from plotting evil. Just five frickin' minutes!

"What?" He demanded. "Can't you see I'm-"

But he recognized the writing on the envelope in his minion's hand.

"...Where did you get that."

"It was next to the door," the eyeless skeleton said. "Someone knocked, then ran."

Nightmare scowled. Why did Dream send him a letter?

He snatched it from him minion, who let out an outraged 'hey!'

The handwriting was definitely Dream's. It was in black ink; a lot like the kind that had been common when they were kids.

A chill ran down his spine. Why would Dream use that kind of ink?

Something was up.

"This was there too," Killer said, pulled out another piece of paper.

"What does it say?" Nightmare asked impatiently.

"'Dream is dead.'"

Nightmare almost dropped the envelope.
This was horribly familiar.

An old memory resurfaced.

An old tradition.

"...No," he muttered.

Killer blinked. "Uh...yeah, that's what it says."

Nightmare ignored him, and tore apart the envelope.

A handful of golden petals flitted to the ground.

Killer questioned it.

Nightmare didn't hear him anymore.

His soul felt heavy.

As much as he'd hated Dream, he recalled how the other guardian had always been trying to cheer him up.

Even his cold soul ached as he realized that, all this time, in contrast to what he'd thought he'd known, somebody had loved him.

And now he was gone.