The time when Finn and Jake teamed up with Marceline to defeat the Lich. It was a really intense battle. Finn showed great courage, Jake was there with his funny remarks and useful powers, and Marceline added her vampire - like strength. Together they managed to stop the Lich from causing destruction in the Land of Ooo.
It depends on your perspective. Berserk is definitely dark, with its violent and tragic storylines. But some might argue there are other mangas that could be equally or even darker based on personal preferences.
One of the darkest Marvel comics is 'The Punisher MAX'. It delves deep into the gritty and violent world of Frank Castle with very intense and disturbing storylines.
Well, for a manga to be the darkest, it often features a lot of hopelessness, a cruel and unforgiving world, and characters facing constant and overwhelming hardships. Also, the art style might contribute, with heavy use of shadows and dark colors.
Hellboy has its dark moments, but it's not unanimously regarded as the absolute darkest. The perception of darkness varies from reader to reader, and other comics might push the boundaries further in that aspect.