The day it all went to hell
Game of Thrones: The Red Wedding leads to the doom of the stark campaign in the war of the five kings
God of War: Kratos kills his family after being tricked by Ares, realising he has lost everything in the pursuit of power
Star Wars: Anakin joins Palpatine after saving him from Windu, dooming the Jedi order in the process
Berserk: Griffith sacrifices the Band of the Hawk during the Eclipse, killing all of them except Guts and Casca to become a Godhand member
Final Fantasy 16: the Night of Flames on which Clive gets betrayed by his mother, the duchess, leading to Joshua/the Phoenix dying and Clive being taken prisoner and enslaved for 13 years