Qamek is a poison used by the Tamassrans on those who refuse to be converted into the ideology of the Qun, including all captured mages who are deemed hopeless by default. It effectively lobotomizes victims, wiping out their memories and rendering them into mindless slaves which Qunari call Viddath-bas.
In banter in 9:37 Dragon between Fenris and Isabela, he mentions that Isabela was very lucky that Hawke didn't turn her over the Qunari because she'd be converted to the Qun. When she asks if she could refuse, Fenris mentions that there is always the qamek which would turn her into a mindless laborer.
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The plot follows BioWare's own canon, meaning that Alistair is king of Ferelden in the comic's story.
Isabela is captured along with Alistair and Varric by the Qunari and imprisoned in the fortress of Akhaaz on Seheron. While she is alone in a cell, Rasaan, a Qunari Tamassran, visits her and attempts to convert her. Along with her, a Qunari guard also entered the cell and places the qamek in the center of the floor. After a lengthy interrogation, Isabela still refuses to embrace the Qun, so an angry Rasaan forces Isabela's head into the qamek telling her that she will forget her names afterwards. However Isabela manages to fight back and does not suffer the effects.
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If the Inquisitor orders Iron Bull to save his Chargers during the quest Demands of the Qun, two Qunari assassins will be sent to Skyhold to deal with him. Before he manages to kill them, one of them stabs Bull with a dagger coated with saar-qamek. Bull, however, had anticipated the confrontation and took doses of the antidote in advance, and therefore was unaffected by it.
Both qamek and lyrium are glowing substances. The loss of memory property also happens to the Templars after they consume lyrium for years, and a lyrium brand is used to render a mage Tranquil in a ritual akin to lobotomy. Whether qamek is composed of lyrium is not yet known.