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.
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?"
Hotfix (2015-06-23):Fixed an issue that could cause Shadow Priests to be unable to shift out of the Shadowform visual effect.
Patch 6.2.0(2015-06-23): Shadowform no longer applies its visual effect to the priest's mount.
Patch 6.0.2(2014-10-14): No longer allows any healing spells to be cast but also no longer has a restriction on casting other Holy spells. [Mind Quickening] is no longer tied to Shadowform and is now a separate passive.
Hotfix (2014-01-08):Shadowform now increases the Priest's armor by 100% (up from 60%).
Patch 3.3.0(2009-12-08): Shadowform will now cause [Devouring Plague] and [Vampiric Touch] to benefit from haste. Both the period length and the duration of these spells will be reduced by haste. In addition, the mana cost has been reduced from 32% to 13% of base mana.
Patch 3.1.0(2009-04-14): Now reduces all damage taken by 15% (previously only reduced physical damage). Bonus damage from critical strike chance removed and replaced by the ability of those periodic damage spells to generate critical strikes.
Entering Shadowform gives a new action bar (like stealth).
Patch 1.7.0(2005-09-13): Switching to Shadowform no longer cancels Holy buffs on a priest.
Patch 1.6.0(2005-07-12): Changed so all spells except Holy can be cast rather than only Shadow and Discipline spells. Tooltip updated. This fixes several bugs that prevented various actions (creating tradeskill items, mining, summoning a mount, using a hearthstone and many more).