Shadowform

Shadowform is a level 10 Shadow priest ability. When used, it causes the priest to assume a Shadowform, increasing spell damage dealt by 10%.

Known bugs
Appearance-altering actions that take place while in Shadowform (such as being poisoned or diseased) will alter the character's appearance until Shadowform is dropped and reassumed.

Trivia

 * Shadowform is considered the defining shadow priest ability due to its visual appearance altering effects.
 * Shadowform is an epic Priest spell in Hearthstone, where it changes the standard Hero Power to deal damage in the form of Mind Spike rather than healing in the form of Lesser Heal. Using Shadowform a second time transforms Mind Spike into "Mind Shatter", which deals an extra point of damage. The flavor text reads: "If a bright light shines on a priest in Shadowform… do they cast a shadow?"