The Wardstone for Asylum is needed to enter Spellhold in Chapter 4. It can be gained from Perth the Adept, who will try to kill you when you enter his house. This item appears in Baldur's Gate II: Shadows of Amn and Baldur's Gate II: Enhanced Edition.
In-game description
| “ | This is the wardstone obtained from Perth the Adept. It should lower the wards that keep you from entering Spellhold. | ” |