Defeating the 9 Valkyries:
When you complete the main story you will unlock the Valkyrie locations. Each of the 8 Valkyrie locations are the domain to a different Valkyrie which you need to defeat in battle. Once you have defeated the eight Valkyries you will have acquired the head of each. Take these heads to the council of the Valkyries and you will unlock a 9th Valkyrie location which will be the domain of the Queen Valkyrie. Even on Normal difficulty setting these are tough battles so it is advised that you attempt them with you best armour and weaopns. You will need to have a chisel in order to gain access to the Chamber of Odin. The main reward for defeating Valkyrie in God of War is Valkyrie armour.
Upgrade the Blades of Chaos:
To upgrade the Blades of Chaos you must use a material called 'Chaos Flame' which you pick up during the story. This is done by getting the blades to level 4 and then in level 5 defeating the Valkyrie in Muspelheim who will upon it's death drop an item called 'Raging Inferno of Muspelheim', which you then need to take back to any shop. At a shop, go to 'Buy', followed by 'Resources' and click on the 'Chaos Flame' to trade it for the item you picked up from the Valkyrie. If you now put the Chaos Flame in the Blades of Chaos you will fully upgrade them. Completing this task will unlock the 'Why Fight It?' trophy.
Amulet of Kvasir:
This amulet is a defensively oriented talisman that focuses on dodging rather than parrying like the Golden Talisman of Protection. When you have the Amulet of Kvasir equipped you will be able to dodge attacks before they happen as time will slow down, similar to Witch Time in Bayonetta. You can get the Amulet of Kvasir early in the game but later in the story than the Golden Talisman of Protection as you need to reach Alfheim and the Lake of Nine.
Getting the Golden Talisman of Protection:
This talisman has several benefits, it enables you to recover faster after being hit by an attack, it increases your defense when you are guarding with your shield and and it increases your parry window allowing you to reflect attacks and counter without requiring super precise timing. You can get the talisman within the first five hours of the game by heading to the Lake of Nine in Midgard and a location known as Stone Falls.