This heavy helm is given to the Warden by Bann Teagan in Redcliffe Village after the attack. However, it is only offered if there are no deaths during The Attack at Nightfall.
Note: Dwyn's two guards are not counted amongst the deaths and will not affect the outcome. Non-descript Militia (labelled "Militia") also do not count toward this total.
Even on Nightmare if a party mage sits in the middle of the mob of NPC allies running Cleansing Aura and occasionally chugging a mana pot there should be 0% chance for any fatalaties. Just sit on the TAB button and position the aura over troops that need healing. No other action is required in this situation. Works for both the upper knight battle (best if you don't tell them about the oil, as the knights seem to like running and standing inside the fire), as well as the lower end battle.