This banner is carried by a Warboss or his bodyguard to show how dangerous the warband is. It is decorated with glyphs declaring the warband's invincibility and mementoes of defeated foes. The banner has an almost religious significance to Orks and they will fight ferociously in its presence. Any mob that has one or more models within 12" of the banner may re-roll their Power of the Waaagh! test when they charge.
Waaagh! Banner
Source: Codex: Orks, p. 36