The Sunspear is a dagger made of blisterwood.
The dagger was originally hidden in Morytania, where it was retrieved by a party of eight people, made up of Vanstrom, Kendrick, Gar'rth, Ben Strainge, Tam and three others. During its retrieval, Tam and the three others were killed by bloodveld, Ben was wounded, and the four survivors were captured by Vanescula Drakan and taken to Tenebra. When Gar'rth attempted to use it on his father, he failed, and Ben and Kendrick were killed. Gar'rth was subsequently imprisoned for his assassination attempt.
When Gar'rth was reunited with his father, Tenebra used the dagger on Vanescula, though she somehow survived. During the battle at the River Salve, Gar'rth managed to stab Tenebra when he was weak, and Kara-Meir finished him off by stabbing Sunspear into the base of his skull.
Should the player speak with Kara-Meir on Tuska's corpse while carrying the Sunspear she claims to still have it, and that it is a dagger, not a sword. She and the player then speculate that both weapons might have originated as parts of the same spear, hence 'spear' in the name of these weapons.