Shadowform does not increase spell damage done by items or consumables.
Shadowform no longer has restrictions on spells that may be cast.
All abilities available to Shadow, including Holy school spells like [Purify Disease] and [Resurrection], can successfully be cast while in Shadowform.
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?"
Patches and hotfixes
Patch 8.1.0 (2018-12-11): No longer reduces physical damage taken (was 10%).
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 5.4.0 (2013-09-10): Now increases the Priest's armor by 60%, but no longer reduces all damage taken by 15%. Now incorporates the effect of [Glyph of Dark Binding].
Patch 5.2.0 (2013-03-05): Now increases the Priest's Shadow damage by 25% (was 20%).
Patch 5.0.4 (2012-08-28): Bonus to shadow damage done increased to 20% (up from 15%).
Patch 4.0.1 (2010-10-12): No longer reduces threat by 30%. Additionally, [Cure Disease] can no longer be cast in Shadowform.
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.
Patch 3.0.8 (2009-01-20):[Abolish Disease] and [Cure Disease] can now be cast while in Shadowform. Additionally, you can now shift into Shadowform while mounted or sitting.
[Spirit of Redemption] and Spiritual Guidance (priest talent) now work while the priest is in Shadowform.
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).