Helm of Balduran is a powerful relic that once belonged to the famous explorer and founder of Baldur's Gate, Balduran. It is one of the most sought after pieces of headgear in the game. Aside from offering protection from critical hits, it also provides a +1 bonus to armor, lowers THAC0 by 1, increases the max hitpoints of the wearer by five points and gives the wearer a +1 bonus for all saving throws.
This helm can be found in Helm and Cloak in Baldur's Gate city. It is connected to the side quest Searching for Balduran's Helmet. Like many items of Balduran, this is very valuable to merchants and they will buy it from you for 7000 gold.
In Baldur's Gate II, it can be looted from Jon's room on the first level of Irenicus' Dungeon. But it can be replaced by other item if you import it from Baldur's Gate I - see Importing a Character for details.