Magog is imprisoned in the empty dimension.
An empty dimension was a "spare dimension" "every TARDIS" carried as standard equipment.
It was entirely featureless, and a conduit could be activated using a button in the TARDIS to imprison someone in the empty dimension for eternity, where they could not die. The Fourth Doctor tricked the Malevilus Magog into pressing the button in the Doctor's TARDIS, which activated the conduit, sucking him into the empty dimension. (COMIC: Doctor Who and the Iron Legion)
Magog was shown to be still imprisoned in the empty dimension when the Seventh Doctor opened a door at random in a corridor in the TARDIS, which was at the time infected by the Mandragora Helix, only to quickly shut it again when confronted by the Malevilus screeching "HELP MEEEE!" Ace thought at first Magog was Mandragora, before the Doctor told her the truth. (COMIC: The Mark of Mandragora)