The Spiral Mark, located in Siberia, is where the Nullifiers were sealed beneath a large symbol of the Blood Spiral by Lord Casterwill. It can only be re-activated through the Willblade, or through Void on the longest night of the year while the Red Comet is on the sky.
History
After Lord Casterwill defeated the Nullifiers, he sealed them in Spiral Mark using the Willblade of his elder daughter. S2E50 After his death, his best friend, later known as the Betrayer, tried to open it again with Void on the longest night of the year. Casterwill's children defeated him and from there on the Casterwill Family's purpose was to never let it open again. S2E31
Lok Lambert and Sophie Casterwill, however, activated the Spiral Mark by mistake, having been led to think that Willblade would prevent Nullifiers from returning to Earth. S2E50 Even though Dante Vale destroyed the Red Comet, the Betrayer revealed that he could actually use his own powers to open it and bring the Nullifiers back. S2E51