Carsomyr

Carsomyr is a powerful, two-handed Holy Avenger sword found in Firkraag's "hoard" in Baldur's Gate II: Shadows of Amn. It's described as one of the most powerful weapons existing on Faerûn, and it's certainly more powerful than most others found before the expansion. It is technically a +5 two-handed sword (greatsword) that grants 50% magic resistance when wielded, can be used to cast Dispel Magic three times per day, does an extra 5 points of damage against chaotic evil opponents and also dispels magic whenever it hits.

In Baldur's Gate II: Throne of Bhaal, Cespenar can combine Carsomyr with the Eye of Tyr to make it even more powerful, increasing its enhancement bonus and bonus damage against chaotic evil opponents both to +6.