Seisyll was born in the Dyrwood, although he didn't stay there for long; he ran away from his modest backwater homestead when he was thirteen, fuelled by lofty ideas of fame, fortune, and culture in the Vailian Republics. Instead, Seisyll was robbed of his meagre possessions as soon as he stepped foot in Old Vailia. With no money, no job, and no means of returning home, he would almost certainly have fallen into the hands of one of the city's trafficking gangs, had a local businessman not taken an interest. It turned out that Seisyll had a talent for not just language retention, but mimicry in general; in just three weeks he had become semi-fluent in Vailian, and was able to present himself as a respectable member of the Republics. He was subsequently offered work as a mancipatio, a legal type of bonded labour which allowed the Vailian Trading Company to hold his contract, or nexum, for the perpetuity of his debt.
It was on business for the Vailian Trading Company that Seisyll returned to Aedyr, investigating a potential land claim for the Republics. However, when his caravan was overwhelmed by a bîaŵac, Seisyll was forced to take refuge in the ruins of Ciliant Lis. In doing so, Seisyll stumbled upon a mysterious and ill-fated Leaden Key ritual which inadvertently left him with the ability to see, communicate with, and manipulate souls. Through this, Seisyll discovered a plot by the Leaden Key to eradicate animancy - the science of studying souls - throughout Eora. Under the direction of his masters, and with the help of his new allies, Seisyll tracked down and confronted the Leaden Key's apparently deathless grandmaster, Thaos ix Arkannon, and put a stop to the Leaden Key's plans.
The Vailian Trading Company were evidently happy for Seisyll to remain in Caed Nua, the stronghold whose title he had inherited as Watcher. Caed Nua was eventually destroyed when the dead god Eothas rose from beneath its foundations, slaughtering hundreds in the process, before making his way inexorably towards the Deadfire Archipelago. The gods promptly tasked Seisyll with finding and stopping Eothas before he can cross the Deadfire, a goal which largely aligns him with that of its native people. Unfortunately for him, the Company has invested sizeable amounts of money and infrastructure into the region with the intention of closing out their competitors - assets which Seisyll was naturally expected to protect. With multiple groups now vying for the soul of the Deadfire, Seisyll is trying remain as neutral as possible, whilst also concealing the true nature of the Company's hold on him.