When the Tenth Doctor's TARDIS brought him to ancient Gallifrey, billions of years before his time he and Cindy Wu witnessed young Gallifreyans being forced into the schism by a group of Time Sentinels. (COMIC: Old Girl)
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The Untempered Schism is similar to the caldera from The Book of the War, which is a place on the Homeworld that acts as a node point where all lines of historical influence converge and one can stare directly into the heart of time.
Jag kan inte hitta några bevis på den otestade schismen i tidigare doktorsverk.