As a specimen for Soul Master’s experiments the Failed Vessel was put through all kinds of torture, from mental to physical. The scientist didn’t treat him like a living creature, but an object with no thoughts or feelings of his own. Even when he would try to escape and did things that something with an actual mind would do the bug refused to think of him as anything more than a mindless thing. It didn’t help that the Pale King seemed to believe the same things about vessels, thinking of them as shells animated by the void. Any indications otherwise were always explained away. Those showing attitudes that contradicted this belief were written off as flawed dolls. This belief, of course, was echoed by most bugs in the kingdom, with the White Lady being the exception. However, not even she could prevent what was happening to the Failed Vessel in the Soul Sanctum. and he was left there for years, used and abused by the scientists that worked there. It wasn’t until the Pure Vessel had become an adult and was close to being prepared to seal the Radiance that he managed to escape, leaving the corpses of his tormentors scattered all over the laboratory. Present times he avoids the place completely, as it will trigger his PTSD and send him into uncontrollable panic attacks. It’s the one place in Hallownest that Ghost could access without the Failed Vessel making any attempts to stop him.