This amulet is the Valknut, with Norse runes embedded throughout the three entangled triangles.
The Valknut is a mysterious symbol consisting of three interlocked triangles, and is thought to be strongly tied to Odin. The name comes from Old Norse for "Knot" and "Slain Warrior", loosely meaning the "knot of those fallen in battle". It is thought that the Valknut was displayed at the burial sights of those who dies in battle as a sign that they were to be taken to Valhalla.
The pendant is 4cm across on a 56cm thin twisted glossy cord with a metal clasp at the end.