Previous universe war

River Song was aware that the previous universe before N-Space was wiped out by a war. Some archeologists theorised that the SporeShips were surviving weapons from this war, although this theory was proven false. (AUDIO: Signs)

By one account, the previous universe was the Great Old Ones' universe, which was destroyed in part by the Gods of Ragnarok. (PROSE: Divided Loyalties)

Behind the scenes

 * Craig Hinton's notes for The Quantum Archangel specified that the Great Old Ones' universe was destroyed in a "final great war" involving that universes' equivalents of the Time Lords and the Daleks fighting over the equivalent to the Key to Time in a time war manner similar to the War in Heaven, with the Gods of Ragnarok involved in the conflict's conclusion. (Shelf Life)
 * The finished novel reflects these ideas via an alternate reality version of the War in Heaven in which Daleks are the Enemy, with this war ending in its universe being destroyed with the possibility of eventually reforming with a new set of physics.
 * The idea of such a conflict is also echoed in the temporal storm alluded to at the end of PROSE: Millennial Rites.