One hundred and fifty years ago, four street children from the Weissenberg docks were identified as having exceptional magical talent. Magisters from six colleges argued over who would take them on as apprentices. It took the Supreme Patriarch to resolve the dispute, with the children apprenticed to the Orders of Life, Shadows, Heavens, and Beasts.
Over the next eight years, their early promise came to fruition. Unknown to the colleges the four had reunited and were working together. They used secrets from each other's colleges and came to be able to work magic that used a combination of winds. They recorded these spells into a grimoire.
The colleges greeted these revelations with panic. It is strictly forbidden for a Human wizard to work with more than one Wind of Magic. Whilst these spells were designed for separate wizards working with separate winds, the advice on such matters is not clear cut. The four were convinced to keep their research quiet but witch hunters heard of their activities. The four disappeared, never to be heard of again.
The witch hunters destroyed all their research and property and ordered their names struck from the records.
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Rumours emerged that a spell designed by the Weissenberg Four, as they came to be known, has appeared on the open market.
[...]
SKIN OF BONE AND BARK
CN: 1 for Ghyran and 3 for Ghur (to be contributed by two different wizards in the same round)
Range: The spell affects either one of the casters
Target: 1
Duration: Willpower Bonus (use the higher Willpower of the two participants) rounds.
Created by the Weissenberg Four, this is a new form of Colour Magic, with two magisters able to combine their power to cast a single spell. While not technically illegal, several interested parties, including the Cult of Sigmar, strongly insist that it should be.
The caster's skin grows rough and hard. The wearer gains +20 Toughness.