At the age of nineteen, Vladislavovich is said to have been part of a cult dedicated to the Dark Gods. He was taken to a temple where he partook in a group ritual admits lit candles, a shadowy pool and statues of writhing gods. While he and the other cult members chanted, their leader performed a sacrifice on a chained prisoner. Presumably to summon a daemon.
At some point in his life, Vladislavovich is said to have tracked down a Sorceress of the Dark Gods. Guided by a Tsuandanese map, he found her in a small cottage, on his third week of travel through a steppe (presumably the Makhar Steppe). Wanting to know how to better control summoned daemons, he questioned the sorceress, but was supposedly given little in terms of answers. She seemed mostly amused by his precesses, but recommended summoning daemons of Nukuja, since they posed the least risk to the summoner. 
Vladislavovich later claims to have left the life of a cultist. He details that life in his work titled Memoirs of a Former Follower of the Dark Gods.
Vladislavovich as a narrator
Vladislavovich appears as a narrator twice so far, both times with extracts from his memoirs.
The extract in T9A:FB Daemon Legions details his encounter with a sorceress, while in T9A:FB Cultists you can read about his participation in a ritual at the age of nineteen.
Neither instance of the extracts are dated, nor do they hint to when they are from. If his claims about his life are true though, his work offers a truly valuable insight into the workings of the cultists.