The Sunsword is a unique blade that was once wielded by Sergei, Strahd's brother. Paultin is the current wielder of this sword.
The Sunsword may be sentient (it says that it is in Curse of Strahd on page 223, but Chris has not depicted it as such).
It has the properties of a sunblade
+2 to attack and damage rolls
deals radiant damage instead of slashing damage.
does an extra 1d8 radiant damage when it hits undead.