In the Underworld series, Viktor commissions a Master stonemason and Metalsmith to design, excavate and build a dungeon underneath Viktor's fortress. Unbeknownst to the stonemason, this would later become William's prison.
Viktor wanted to eliminate all evidence regarding the building of the prison and decided to kill the stonemason and his family so that nobody could ever be persuaded to tell Lucian or William of the whereabouts of the prison.
Viktor was so obsessively afraid of Marcus finding out where his brother was hidden that he even lied to the men who helped him in executing the families of the dungeon's builders, telling them he was merely in need of blood. Viktor tracked down the head builder and slaughtered his family with the aid of Soren and Kraven.
It is revealed that the stonemason is Selene's father and construction started on the prison when Selene was approximately 6-7 years old. Viktor kills Selene's family and turns Selene into a vampire when she was 19 years old.
If Viktor was so worried about anyone finding out about the whereabouts of the prison, why did he wait so long to kill Selene's family after construction was complete? Or, did construction on the prison just take a long time causing construction to continue for approximately 12-13 years?