You could see Apocalypse only on the test server. More informations from TibiaWiki:
Apocalypse - was once the strongest monster in Tibia. He is a Member of The Ruthless Seven, The Total Destruction. Apocalypse's throne is in the Pits of Inferno. He was summoned three times by CipSoft members on Test Servers (May 2006, November 2009 and July 2012). The last time it appeared it was slain, but unfortunately dropped no loot
Apocalypse seen in the Test Server of the Autumn Patch 2009.
First time killed on Test Server ofthe Summer Update 2012