Spoiler warning!========================================================
You can get it:
1. From the chest in the Crypt (Hall of the Dead). The most west chest with the mana stone. The key has 50% chance to appear in the loot.
2. From the Ghost of Gorn (a unique monster) (12.5% chance).
3. From Skeleton Lord, Spirit or Wraith (2.5% chance).
They can be found at:
{13} Myrmideon Forest and Lesser Wilds
{22} Witch Cave, Giants Cave and Sphinx Cave
{26} Astral Dominae Tomb (Teleporter Levels and the Tomb)
{25} Brombadeg Watery Cave
{24} Crypt (Hall of the Dead and Chamber of Gorrors)
{05} Road from Munkharama to Ukpyr
{21} Ukpyr
{24} Crypt (Hall of the Dead and Chamber of Gorrors)
{23} Munkharama Undergrownd Temple
{22} Witch Cave, Giants Cave and Sphinx Cave
{27} High Mountains