A Great Sigil Stone was a piece of Daedric magic used to anchor a Great Oblivion Gate to the mortal realm. A larger and more powerful variant of the traditional Sigil Stone, Great Stones could only be used in tandem with three other Sigil Stones, requiring three lesser Oblivion Gates to be opened in order to produce a Great Gate. The mystical counterpart to a Great Sigil Stone was a Great Welkynd Stone, whose power contained the essence of the mortal plane, while the Great Sigil Stone's essence was of Oblivion.
A Great Sigil Stone was used to open a Gate to attack the Cyrodiilic city of Bruma and to destroy the city of Kvatch during the Daedric Invasion of Tamriel. The stone was recovered and the Gate closed by the anonymous Hero of Kvatch as part of future-emperor Martin Septim's plan to open a portal to the Paradise of Mankar Camoran.