Sadahshaya (The Sundering) refers to the exodus of S'task and his followers from Vulcan.
Sadahshsu'la - (The Sundered) refers to The Eighty Thousand leaving the planet.
Seheikk'he - (The Declared) name S'task and his followers chose for themselves, eventually becoming Rihannsu
Surak and S'task
S'task records something of their first meeting in the memoirs he left before he went off-planet.
Surak looked up from his writing as the young man came in and put down the fruit he was eating. "Who are you?" he said.
"S'task," he said.
"What can I do for you?"
"Teach me what you know."
S'task says that Surak put the fruit down and said to him most sincerely, "I thank you very much indeed. Please leave." "But why? Have I done something wrong?"
"Of course you have," Surak said, "but that is not the point I am making. You are about to get into a great deal of trouble, and I would save you that if I could. Entropy will increase."
"It will increase anyway, whether I get in trouble or not," S'task said.
Apparently it was the right thing to say. "You are quite right," Surak said, nodding. "That is why you should leave."
"You are not making a lot of sense," S'task said, somewhat nettled.
"I know," Surak said. "Logic is a delight to me, but there are things it is no good for." And he shook his head regretfully. "But I must cast out sorrow," he said. "And you, too. Please leave."
S'task thought he would stand his ground, but a few seconds later, he says "I found myself sitting on the pavement outside the front door, and he would not answer the signal. I never met anyone that strong, from that day to this. But I was determined to work with him, so I sat there. For four days I sat there - there wasn't a back door to his apartment - and I was determined to catch him as he went in or out. But he did not go in or out, and I became very angry and decided to leave. Then I thought 'What am I doing sitting here, being angry at him, when I cam all this way to learn how not to be?" So I sat there longer. I don't know how long it was: it might have been another seven or ten days. And finally someone came in from the street and stood over me, and said, 'What about windows?'It was he. He opened the door, and we went in, and I stayed and I studied with him for the next three years."
The Duthulhiv Pirates
By the time of Surak, the first landing on Vulcan's sister planet T'Khut was already centuries in the past and mining expeditions to the inner worlds of 40 Eridani were becoming, if not commonplace, at least not unusual.
By and large the Vulcans regarded the rest of the universe with interest and expected to find - or be found by - other intelligent life at some point. As a result of Vulcan culture in which the threat is usually your neighbor, but the stranger is offered hospitality, they expected to treat fairly and courteously with any outworlders, though always from a position of strength.
First Contact
After having surveyed the system for months, the Orion pirates employed their usual method of subterfuge: stumbling transmission of codes by conventional radio (which according to their own records "no one ever cracked as swiftly as the Vulcans did. It was almost as if they had been expecting it.").
When communication was established, the pirates offered peaceful trade and cultural opportunities; the first messages were debated for months around the planet. Several wars or declarations of war were in fact put on hold or postponed in order to present a united front.
At the agreed anding place near Shi'Kahr five hundred twenty three of Vulcan greats gathered to greet the strangers - but what met them were phasers who stunned those to be held for ransom or sold as slaves, and blew to bloddy rags those who fought or tried ro escape.
Ahkh
What is still called "The War" by the Vulcans, thus demoting all others before to the rank of mere tribal feuds, swiftly took battle to the Duthulhiv Pirates. Although Vulcan never had thought to arm their starships, their chief psi-talents and technicians quickly taught the pirates that weapons aren't everything. Metal came unraveled in ship's hulls,; pilots calmly locked their ships into suicidal courses, unheeding of the screams of the crews.
By a twist of fate Surak had been delayed at the port facility of ta'Valsh, but S'task was at the meeting; and one of those taken hostage. After uttering the famous "I am Vulcan, bred to peace" it was S'task who proceeded to break his torturer's back, organize the in-ship rebellion that cost many slavers' lives, broke into the database and - after safely releasing the other hostages - crashed the luckless vessel into the pirate mothership.
He was picked up weeks later, after much anguished searching, in a lifepod in an L5 orbit; half dead of dehydration but clinging to life through sheer rage. They brought him home, and Surak hurried to his couch-side - to rebuke him. The words "I have lost my best student to madness" are the beginning of the Sundering of the Vulcan species.