The Grimhold is an inescapable prison that resembles a nesting doll and one of the many magical items in the 2010 Disney fantasy movie The Sorcerer's Apprentice. It has a total of 4 layers, each designed to capture one person. However, the innermost layer holds two people, Morgana le Fay and Veronica Gorloisen. This is possible because Morgana's spirit was inside of Veronica's body because of the Human Fusion Spell she cast on her when they were trapped.
Morgana was the first person to be locked in the Grimhold. Every evil spellcaster after her was trapped in the doll trying to release Morgana so she can complete a spell known as The Rising, an accursed magic ceremony that will rise all the evil sorcerers from beyond the grave, and create a perfect army for Morgana to subjugate mortal humans and conquer the Earth.
As seen when Balthazar uses the Grimhold to seal both Veronica and Morgana, when the Grimhold is empty, it's surface was originally blank. However, after being used, the image of the prisoner inside will manifests on it's surface, reveals who was sealed inside. If two people sealed simultaneously inside a Grimhold however, both of their faces separately manifests on sides of Grimhold.
Prisoners inside the Grimhold cannot able to escape from inside, which the only way to escape was by having a sorcerer/sorceress open them from outside. But to do so, a wizard or a witch who wanted to release the prisoner requires an immense power due to magic in the Grimhold was immense. This is why Maxim Horvath the magical life-draining curse known as the Parasite Spell to his own allies once they give what he wants in order to became powerful enough to open the whole layer.
When the Grimhold used on the victims, their body dissolved into dust before sucked in. When they became released however, they manifests as a mass of disgusting organisms and/or fluids before their body fully restored. This can be seen when Dave Stutler in his childhood accidently release Horvath, Horvath manifests as a mass of cockroaches which merged into his true form. This does same with Morgana and Veronica, as when they freed, their body manifests as mass of mud and snakes, while Sun Lok manifests as a mass of flying bugs (possibly a mass of butterflies). But in Abigail Williams's case however, is not unclear how she manifests once released, as she was freed off screen, though the possiblity is that the energy manifesting her escape is fire which symbolically represents her dark legacy as the great witch responsible for the infamous Salem witch trails of 1680.
The contents of the Grimhold are as follows (starting from the innermost layer):
Layer 1: Morgana le Fay and Veronica Gorloisen
Layer 2: Abigail Williams
Layer 3: Sun Lok
Layer 4: Maxim Horvath