Harbinger

Harbringer is an enchanted two-handed sword +3. There is a 5% chance that a hit from this weapon will also cast the spell Fireball with a hit dice of 6d10 upon the target. The sword can also turn ogres into stone if they fail a save vs. spells. The weapon inflicts 1d10 +3 points of slashing damage upon the target.

It also has -3 THAC0 bonus, a speed factor of 7 and it weighs 10 lbs. This item appears in Baldur's Gate II: Shadows of Amn and Baldur's Gate II: Enhanced Edition. It can be bought from in Ust Natha stores

In-game description
"Two Handed Sword +3: Harbinger When the hero Trueblood destroyed the Harbinger, a demon gorging itself on the blood of peasant folk near Luskan, he bound its essence in the only vessel capable of holding it, his fabled sword, Deliverance. Though the evil creature was dominated by the will of the semi-sentient blade, it was not truly defeated, and it transformed the weapon from within. Now it acts much like any other enchanted weapon, but when it strikes, there is a small chance that a fireball erupts, a sign that the demon within still rages at its imprisonment. There is also a chance that any ogre it hits will be turned to stone. Whether this is an ability of the demon or a vestigial power of the original blade is not known."