The Sound of Drums
In "The Satan Pit" (2006), the Beast identifies the Tenth Doctor as "[t]he killer of his own kind." In "The Sound of Drums" (2007), the Tenth Doctor confirms his involvement in killing all of the Time Lords when he says that he was the only one that could end the War and that he had tried "everything". In "The Stolen Earth" (2008), the Tenth Doctor reveals that the entire Time War is "time locked", which means it is theoretically impossible to enter.