Source: Warhammer Armies Project: Unofficial 9th Edition

Plague of Rust
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Spell Level 1Casting Value: 7+

At the Wizard's command, the armour of the enemy begins to rot, shedding away in clouds of tiny flakes.

Plague of Rust is a hex with a range of 24". The target's armour save (excluding Natural Armour) is lowered by one point for the rest of the game (e.g. a model with light armour and shield will only have a 6+ save). Plague of Rust can be repeatedly cast on the same target, reducing its armour save by a further -1 each time. The Wizard can choose to extend the range of this spell to 48". If they do so, the casting value is increased to 9+.