There were many different symbols associated with Norse mythology. Below are some of them and what they meant:
Valknut

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The Valknut was the symbol of Odin. It was the symbol of death, of fallen warrior's spirits passing into the halls of Valhalla.
Horn Triskelion

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The Horn Triskelion symbolised the three drinking vessels that contained the mead of poetry. According to the Poetic Edda, whoever drank the mead would become a poet. This was why the symbol of the three horns represented wisdom and inspiration (especially poetic inspiration).
Mjolnir, The Hammer of Thor

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Mjolnir was a symbol of strength and protection. However, the hammer itself was more than just a weapon. Whilst it was responsible for crushing the skulls of many giants, it was also a symbol of purity, of leading lost spirits out of chaos and into the sacred realm.
Aegishjalmur, The Helm of Awe

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In battle, the Aegishjalmur would strike fear into the hearts of enemies. This is why some Viking warriors used to paint this symbol on their foreheads.