Diablo/Arkaine's Valor Armour

At catacumbs level 5, read the Book of Blood that will talk you about fountains, bloodstones and stuff.

Simply go through the door and beware for those fat ones. Kill them and grab the bloodstone. Put it on the pedestal. Do it two more times. The pedestal must hold three bloodstones.

A wall will magically desappear and a hell will be shown to you. Go through it and kill more fat demons. On the floor you will see the unique item: Arkaine's Valor Armour.

This armour is pretty good at the beginning, since it gives nice magical bonuses.

 Curiosity : before finishing the quest, if you back to town and speak with the townsfolks, they will tell you that Arkaine was a noble warrior from Leoric's navy.