Hreidmar's Cursed Armor can be found in the Hodda Shelter in Vangrinn, near the Blacksmith. This armor is cursed and will slowly kill you if you use it. First, you must complete the five Jotun Blight artifacts located in Svartalfheim to unlock five clarity runes. Then, socket a clarity rune to each piece of the cursed armor to negate the cursed effect. The achievement may unlock here, or you may need to equip the armor set as I did.
Timestamps
00:00 - Intro
00:59 - Jotun Blight 1/5 (Gullnamar)
02:27 - Jotun Blight 2/5 (Svaladal 1)
04:07 - Jotun Blight 3/5 (Svaladal 2)
06:28 - Jotun Blight 4/5 (Svaladal 3)
07:50 - Jotun Blight 5/5 (Vangrinn)
09:29 - Purifying Hreidmar's Cursed Armor