Not Available in Any Stores
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Not Available in Any Stores is a Tome of Knowledge Bestiary entry for Bat, Giant.
Instructions: Order & Destruction
Kill 'Deathwing', a level 40 Champion Giant Bat, in the Chaos Wastes in the Caverns of Terror at 44.6k 3.5k. He can be found in the room furthest back. He will drop a Pocket Item 'Deathwing's Fangs'. Equip it will trigger the unlock.
Once the unlock triggered, you can claim the jewel buying it from the correct merchant in Library (Order) / Lyceum (Destruction).