The Grimhold is an inescapable prison that looks something like a Russian nesting doll. It has a total of four layers, each designed to hold one person. It appears as though the deeper the layer, the more power it takes to open it.
Some people think that one is not conscious of being inside the Grimhold, but since Horvath and Balthazar were conscious of being inside that Han-era urn, we can assume that Veronica and Morgana had a very boring 1300 years inside of it. - edited from