Codex: Chaos Daemons (2013), p64 — Icons & Instruments

Plague Banner

A pervasive aura of pestilence surrounds this icon, causing rusted blades to weep with even fouler and more toxic diseases.

A plague banner is an Icon of Chaos. Once per game, at the start of the Fight sub-phase, the unit can declare it is using the plague banner. If it does so, its melee weapons have the Poisoned (2+) special rule until the end of the phase.