At some point, Emmanuel was cast out of warlock society and forbidden to practice magic by the Spiral Labyrinth when they turned against him. In order to make money so that he could feed and clothe himself, he began taking any jobs offered to him. He gained a reputation as someone who would take even the lowliest of hires.
Sometime around 1879, Tatiana Blackthorn put out word that she was looking for a warlock to put the protection spells customary of a young Shadowhunter on her baby as she did not trust the Silent Brothers or Iron Sisters to do it. Gast was the only one who was willing to get involved and agreed to performing the spells. Before he got there, however, he was intercepted by Belial who demanded that he place a piece of his essence in the child's soul as an anchor while her does the other spells. Though he did not want to do it, he was threatened into obedience and complied.
Years later, in 1896, he was ordered by Belial to preserve Jesse's body, as the boy had died when receiving his first rune—as a result of the rune having a bad reaction with the demonic essence inside him. Gast did as he was asked, keeping the boy in a twilight state where he was neither alive nor dead, and his ghost awaking at night.
In 1903, he accepted a job from a cloaked individual—which turned out to once again be Belial, though he may not have known it at the time—in which he had to raise a Mandikhor demon. In raising it, he used dimensional magic in order to make the demon impervious to sunlight. He then captured it in a Pyxis box to keep it safe and give it to the person who hired him. However, when he returned home with it, he was betrayed: Belial killed him and released the demon from its confinement.
Some months later, Lucie called on his spirit once more, this time to question him about Jesse's death. Though Gast willingly told her of having cast the spells, he tried to hold back sharing about the anchor as he knew it would anger Belial and there were still terrible things he could do to him. However Lucie continued to press for it, holding him while Belial compelled him opposingly. He was forced to confess by Lucie's powers but was immediately torn apart; his ghost splitting down the middle and his tattered pieces sinking into and staining her floor.