Hell mode can be accessed after defeating Hanin and deciding to Save Perna by entering the red portal in Dubaq Clan. In Hell mode, all levels of demons have been increased DRASTICALLY. As an example, Varians in hell mode are at lv72. Hell mode is the perfect place for leveling. Still in Hell mode, Monsters' drops, Herbs/plants, and also Ores, are different from normal ones, so its time to go for collect 'em all again. Monsters now drop higher level weapons/armors and souls,however,their drop rates are very low. There are also hell mode specific ores and herbs, which have very low collection rates. So it takes time to collect them in large quantities.

The things which are not changed in Hell mode are the characters, sold weapons and armors or medicine, and also Hunter points and money in completing Hunter's quest.

Bone from Hell All monsters in kamael
Book from the hell All monsters in Distia (?)
Dust from Hell All monsters in Inferno
Ghost stone All dungeons
Fingernail of the hell Arbernus (essence of destruction 5)
Rope from hell All monsters in Gehena
Sulfur Mushroom Arbenus (essence if destruction 5)
Emblem of the Hell Tower of Punishment

This list is incomplete. If you have any further information, please contribute.