This is a large Thor's Hammer pendant with the same design on both sides on a chain with a very detailed design in the Urnes style.
The Urnes style is the last phase of Viking art. It takes its name from the remarkable carved wooden doors of the stave church at Urnes, Norway. The style is a refinement if the Ringerike style and depends upon interplay of gracefully curving lines for its effect.
There are three dragons primarily represented in Norse mythology, Nidhogg (or Nidhoggr), Jormungand (or Jormungandr), and Fafnir.
You'll find 3 dragons represented on this pendant as well as a triquetra symbol and Thurisaz rune.