Retrieve the 5 Sigil Fragments from the defenders in Stromgarde, and bring them to Tor'gan in Hammerfall.
Thoras Trollbane's tomb is protected by three sigils--the first of which you retrieved from the Syndicate. The other two will be harder to get, as they're held by the firmly entrenched Stromgarde militia.
The sigil of Thoradin was divided into eleven pieces by Thoras' son, Galen, and dispersed among the Stromgarde defenders. Retrieve the pieces and bring them to Tor'gan, the troll shaman, he will be able to bind them into the complete sigil.