Head

The Heads of the 4 Caste Overlords are required to enter Moloch's Step in Stonehenge.

The Nicor is the caste Overlord in the Spectral caves of the Torment.
Oriax is the caste Overlord in the Necro realm of the Lair of the Damned.
Ash is the caste Overlord in the ? caves of the ?
Errgoth is the caste Overlord in the ? caves of the ?.

http://www.diversityj.com/AB/HGL/Items/HeadOfNicor.jpg
http://www.diversityj.com/AB/HGL/Items/HeadOfOriax.jpg

The Caste Caves
These mystic caves can be found in the wilderness areas outside of Stonehenge. Each caste of demon - Beast, Necro, Spectral, and Primus – has a powerful overlord that resides in these caves. The caves themselves are sealed by the essential aspect of their master’s caste. These caves cannot be entered until the essence of five (5) Champions of the matching demonic caste has been collected. Essence can be collected from any Champion, so adventuring above and below the streets of London may gain you the keys to unlocking these special areas.

Once a character has gathered the required essence of a single Caste, they can either solo or lead a party into the caves and battle against the resident overlord. Not only are there great and special riches to be won, but the head of the caste overlord is also claimed. Once a character has collected the heads of the Beast, Necro, Spectral, and Primus overlords, the adventure continues.

(description taken from the Stonehenge Chronicles Patch Notes)