Written for the prompt: Mycroft has seen some awful things, over half of them due to his own orders, but the worst thing he witnessed was his brother having an awful seizure one day in his office, or the Diogenes club.
And worst was watching John immediately take charge of the situation whilst Mycroft just could only stand there, horrified.
It is completely up to the author why Sherlock suddenly has a seizure, all I ask is that it is set during the time of the series and it is his first seizure.

Mycroft Holmes had seen a number of things in his lifetime, far too many of them caused by his own hand, sometimes awful things that would have filled his nightmares, had he been able to sleep. Many of those that he's had a hand it weren't quite necessary, perhaps could have been avoided, but all of them were decisions within his control. And sure, some of those decisions were horrifying, but Mycroft just grit his teeth and beared it. They were awful decisions, but someone had to make them.

Perhaps it was why he couldn't comprehend awful things that weren't within his control.

Sherlock was in his office, upset about his interference in a recent case. Mycroft certainly wasn't going to allow Sherlock to become involved in matters of international relations, largely because his people skills were so dreadful.

Sherlock hadn't taken kindly to the news he'd been left out of a double murder, simply because his brother felt that way.

"Completely unacceptable," Sherlock hissed, stopping his pacing to glare at his brother.

Mycroft only regarded him coolly.

John watched both of them warily.

"It's one thing if it was something completely dull, but this was a double murder in a locked room, Mycroft," he hissed. "Not at all dull. You should have let me help!"

"Sherlock, are you alright?" John asked. Mycroft glanced at him. The doctor seemed concerned, which was unusual. If Sherlock was just in a mood, then John would have already been aware of that, and thus, been unfazed, but he was, which meant it was something else.

"Of course I'm fine," he snapped, but it wasn't absolute. There was a hint of something else in his voice, something that Mycroft raised an eyebrow upon hearing. Doubt.

John took a closer look. "You're flushed," he noted. "Sit down and let me look at you."

Sherlock scowled, but allowed John to push him into Mycroft's chair, his knees bending almost shakily as he collapsed into it.

"I'm fine," Sherlock repeated. John wasn't convinced.

He examined Sherlock's eyes as he kept a finger on his pulse.

"Your heart rate's fast, but your breathing is slow. Are you sure you're feeling alright?"

Sherlock pushed him aside and got to his feet. "Of course I am," he grumbled.

He only made it two steps before a different expression crossed his face, and he lunged for Mycroft's wastebasket, gagging into it.

"Still sure about that?" John asked kindly as Sherlock wiped his mouth on his sleeve.

Sherlock shot him a look. "Can we just go home?" he muttered.

"Of course," John said. "Another time I suppose Mycroft."

Mycroft nodded, the worry concealed by a tight smile on his face.

"Do take care."

As John nodded at him, Sherlock straightened up, only for a second, before he crashed into Mycroft's desk and fell to the floor.

John was by his side in a second, even before the tremors started.

"Damn it!" he swore, pulling Sherlock away from the heavy desk as his limbs began to jerk wildly.

Mycroft was frozen by the sight of his brother, moving unnaturally on the floor. He had no clue what to do. His mind could only spin through scenarios of death and destruction, as it was often commanded to do in other situations. But not here. Not with Sherlock.

John's shouting brought him back.

"Mycroft," he called exasperatedly. It must not have been the first time. "You need to call an ambulance."

Mycroft nodded, jabbing a button on his phone.

"Sir?"

"Call an ambulance Anthea," he ordered.

"Of course. For your brother?"

"He's... having a seizure," Mycroft managed to say, swallowing hard.

"Noted," she replied, and hung up.

Mycroft stood there and watched as John stripped his sweater off and tucked it under Sherlock's head.

"Mycroft?" John said again, more gently this time. "I need you to help me."

Mycroft shook his head. Him help? No, he'd be more harm than good at the moment.

"Come here," John urged. "I need you to help me roll Sherlock on his side in case he throws up again."

Mycroft nodded, his feet shuffling towards his brother, still spasming on the floor.

"Pull him towards you," John ordered, his voice calm and commanding.

Somewhere in the back of Mycroft's mind, it registered that John must have been an excellent doctor. Calm in a crisis, and yet completely capable.

Mycroft obeyed, following John's instructions until Sherlock was positioned on his side, his head cushioned by John's sweater and Mycroft between him and the desk.

John told him the only thing left to do was wait.

Mycroft wasn't one for waiting, especially as he watched his brother convulse on the floor, his eyes rolled back in his head.

It was the most awful thing he'd ever seen.

Sherlock hadn't stilled by the time the ambulance got there, and swept him and John away.

Mycroft politely declined, citing work as the reason he couldn't go.

But it wasn't that. It was because it hurt to see Sherlock like that, and he wasn't sure if he was brave enough.

"Camphor toxicity," John informed him wearily the next day when he went to visit. Sherlock was asleep, although Mycroft suspected it was not of his own accord. "Probably an attempt to poison him. It's used in sweets, which is probably how he ingested it."

Mycroft nodded. "I'll take care of it," he assured John.

He watched his little brother for a while, wonderfully calm and still. He felt a sudden rush of appreciation for John Watson, this man so calmly sitting before him despite what had happened.. The unassuming man that Sherlock had chosen to share his life with. As far as Mycroft was concerned, he couldn't have chosen anyone better for the job.

"Thank you John," he said quietly.

John looked up from the book he was reading and smiled.

"No problem."