Ring of Hotek

The Ring of Hotek is a magical trinket that not only protects against hostile sorcery, but punishes such casters with a greater chance of miscasting.

History
Hotek was an outcast priest of Vaul. It was he who forged Malekith's rune-etched armour, and oversaw the creation of many of the Witch King's most terrible weapons. Renegade and turncoat though he was, Hotek was no fool. He knew that he lived only at Malekith's fickle pleasure, and so created for himself an armoury of defenses--both magical and mundane--with which he hoped to blunt the Witch King's wrath long enough to escape. Ultimately, Hotek did not perish at Malekith's hand, but was poisoned at Morathi's order. His armoury was broken up soon after, the scattered pieces changing hands with the ebb and flow of patronage and power. None of the artefacts have known as many masters as the Ring of Hotek, for this trinket of obsidian and black diamond is claimed o be the renegade priest's greatest work.

Source

 * Warhammer Armies: Dark Elves (8th edition) -- p. 63