Collapse of the Pillars

The Collapse of the Pillars was a seminal event in the history of Nosgoth. It signalled the end of the Circle of Nine as a force in Nosgoth as well as effectively 'sealing' the corruption of the Pillars. After the collapse, the health of the land of Nosgoth itself began an inevitable decline, slowly becoming a wasteland.

History
With Ariel's murder in Blood Omen (at the hands of the Dark Entity) leading to the corruption of the pillars, Kain was sent on a quest to restore the Pillars by murdering each of the corrupted guardians in turn and returning their pillar token; thus restoring each pillar. When Kain fought the final guardian,Mortanius, and apparently defeated the Dark Entity possessing him, Ariel appeared and revealed to Kain that he was the last guardian (born to replace her asbalance guardian) and that to restore the pillars, he must sacrifice himself; presenting him with a choice to sacrifice himself and restore the pillars; or refuse the sacrifice and likely rule the damned Nosgoth. Realising that, as the last vampire in Nosgoth, sacrificing himself would mean the elimination of his species, Kain rejected the sacrifice. Unable to take the strain, the Pillars 'collapse' and Nosgoth was damned.

The collapse has severe effects, initially the resulting 'explosion' sends out a 'psychic shockwave' through Nosgoth (as seen in Defiance) and creates a rift which "breaches through the dimensions", (a rift which in the fourth timeline allows the Sarafan Lord to reach the Material Realm and set up the Hylden Gate - setting up Blood Omen 2). The corruption of the pillars is sealed into the land itself, leading to a slow but inevitable decline with much of the vegetation dying within a few centuries and the land a desolate wasteland by the Soul Reaver era.