Canon Background
https://awoiaf.westeros.org/index.php/Qyburn

In-Campaign
Qyburns amoral and curious nature had taken him down a dark road by 285 AC. He had secretly begun to experiment on living, healthy, and unwilling people to expand his knowledge of “the higher mysteries”. A young maester, Thaddeus, on sabbatical at the Citadel discovered Qyburn’s secret inadvertently and alerted Arch Maester Ebrose. The pair barged in on Qyburn and discovered his hands covered in the blood of an unfortunate soul. A conclave was convened and all, even Marwyn, agreed to strip Qyburn of his chain.

Months later, in Lannisport, Qyburn was a house guest of The Duke when he spotted Thaddeus approaching with his fellows from House Jasper from an upstairs window. Thaddeus also spotted Qyburn and abruptly broke of his approach and headed back into the market, disappearing in the crowd.