Shathol'Syn is the ritual suicide committed by the four losing Amarr Heirs following the election of a new Emperor. The tradition arose following the Moral Reforms, in order to minimize the risk of conflict between the new Emperor and his old rivals. The tradition has only twice been broken.
Origin
Prior to the Moral Reforms, the Amarr Emperor was considered the first among equals on the Council of Apostles. The Emperor was more or less a figurehead of power, chosen from among the Apostles by the other Apostles, and in practice wielded essentially the same secular power as his fellows. The Moral Reforms changed this, placing increasing religious and secular powers in the hands of the Emperor, making him the undisputed leader of the Empire. The Privy Council, which replaced the function of the Council of Apostles, were subordinate to the Emperor and was composed of the other claimants to the throne.
It was quickly realized that when the Emperor died, those claimants who lost out would pose a threat to the new Emperor's power. Supporters of the losers might cry foul and oppose particulars of the selection process, or push their chosen to oppose the new Emperor. Opposition would cause strife within the Empire at best, outright civil war at worst.
Many solutions were proposed, but all had flaws. It was finally the head of the Ardishapur Family that proposed the ritual suicide. Drawing on old and then-obscure Scripture, the suicide had a religious precedent and would neatly solve all the problems. The losing claimants would remove themselves entirely from the picture. Their deaths would give the Emperor time to consolidate his power, much as the new Heirs would be consolidating their own.
The Privy Council agreed that this drastic solution was the only way to ensure that the Empire would not risk its own destruction with ever new Emperor.
Breaks of Tradition
Shathol'Syn has only been broken twice. The first was by Garkeh Khanid, the heir of the Khanid Family, following the election of Heideran Kador as Emperor. Garkeh had only recently been made the Heir of Khanid Family and was a young, ambitious man. A powerful Heir, he was considered the favorite to be elected Emperor. When he lost, he refused to commit suicide, stating,
I will not be ordered ... to destroy myself when my work is unfinished. You will not take anything from me, ... least of all my life.
Instead, Garkeh declared himself Khanid II, King of the Khanid Kingdom, and seceded from the Empire. Though this was the very thing Shathol'Syn was meant to avoid, little warfare broke out, as Heideran VII and the new Heirs were busy strengthening their own positions, leaving them unable to counterattack. The Empire invaded later, but by then the Kingdom had dug in, making it virtually impossible to conquer.
The second time was in YC110, by Jamyl Sarum. Believed dead following her loss of the Succession Trials to Doriam Kor-Azor in YC105, Jamyl returned from the dead to lead the Empire in defense of the invading Elder Fleet. Jamyl had appeared to commit suicide following Doriam II's coronation, self-destructing her ship. Because of the doctrine of Sacred Flesh, it was forbidden for Jamyl to clone herself. The Sarum Family declared her return a miracle, while her control of a mysterious super weapon and widespread popularity made it difficult for others to challenge her. Eventually, she was crowned Empress and cleared of wrongdoing by the Theology Council. There were still elements within the Empire who believed Jamyl to be a heretic and false Empress, though these kept their voices muffled for fear of reprisal.