There are thirteen Dark Altars in different parts of all three Realms. Priestesses are assigned to take care of these sacred places. Some, but not all, of them know how to open the Gate. Especially in Terreille this knowledge has mostly been forgotten.
Every Dark Altar holds a Gate, a pathway to another realm. The gates are opened by igniting thirteen candles in right order. If the one igniting them doesn't know the right order in which the candles are supposed to be ignited, the Gate might not open or it will open to some other place than the intended one.
In Daughter of the Blood the Gates are used rarely, mostly by Saetan SaDiablo, but in Heir to the Shadows and onwards, after the existance of Kaeleer has made known to the people of Terreille, the use of Gates, both legal and illegal, increases.