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The Decamillennial was the ten-thousand year anniversary of the signing of the Guildpact on the plane of Ravnica.

On the day of the Decamillennial, the Selesnyan leaders (the Chorus of the Conclave) would perform the convocation, a ritual through which the entire plane would become blessed, their sins forgiven.

However, the vampire Szadek attacked the ceremony and, with the aid of his lupul and Savra, he almost destroyed the Guildpact, until Agrus Kos and others managed to arrest him (though this in itself broke the Guildpact).[1]