In the first half of the 5th century BCE, a group of freedom fighters comprising of the Persian elite and led by Artabanus opposed the secret rule of the Order of the Ancients over the Achaemenid Empire. Artabanus' group gathered with the goal of eliminating King Xerxes I of Persia, an ally of the Order of the Ancients.
Despite meeting a terrible end, with the group splintered and Artabanus on the run under the alias of "Darius", their values were transmitted through Darius, and thus, to the Assassin Brotherhood.
In August 465 BCE, the group, including Artabanus, Amorges, and Pactyas, ambushed King Xerxes I and his entourage while they were travelling. While the group distracted and eliminated the king's guards, Artabanus himself assassinated the king using a Hidden Blade he had created especially for the task. This would become the first assassination using the Hidden Blade, performed centuries before it became the Assassins' signature weapon.
After Xerxes' death, Artabanus sought to prevent the Order of the Ancients from exerting their influence on the king's successor, the young Artaxerxes, and planned to kill the new king before he could be manipulated. He faced opposition from Amorges, who instead persuaded the rest of the group to join forces with the Order and use their combined resources to create a better future for Persia. The group completely disbanded when Amorges turned on Artabanus during the latter's attempt on Artaxerxes life, which forced Artabanus and his family into exile.
Sometime later Artabanus, now having adopted the name of "Darius", made a second failed attempt on Artaxerxes life using poison, which left the young king blind, before spending the next few decades being pursued by his former allies.
For the parallels with the later Assassin Brotherhood, their leader Darius came to be seen by the Assassins themselves as one of their predecessors with him being honored posthumously among other illustrious members of the Brotherhood.