Roranach's Horn is a helmet which offers the wearer 50% resistance against damage from crushing damage. It can be used by Cespenar to upgrade the Staff of the Ram.
This item appears in 
Baldur's Gate II:
Throne of Bhaal (2001)
This icon indicates content from the original Baldur's Gate II: Throne of Bhaal campaign. and 
Baldur's Gate II: Enhanced Edition – Shadows of Amn (2013)
This icon indicates content from the Shadows of Amn campaign of the Baldur's Gate II: Enhanced Edition.(ToB).
Location
It is dropped by Gromnir Il-Khan when he is killed in the first chapter of Throne of Bhaal.
In-game description
| “ | This helm is carved from the horn of a great ram. Etchings on the horn suggest the beast was once worshipped by ancient barbarian tribes. | ” |