Hand of Reckoning is a spell. It can't be cast when silenced, but based off testing on Growl and Dark Command, no longer suffers from miss chance when cast by a level 80 vs a boss-quality mob. Prior to Patch 4.0.1, it had a 17% chance to miss raid bosses (in contrast to the 8% chance to miss boss mobs for melee attacks). Prior to Patch 4.0.1, even if you were melee hit-capped (263 hit rating at level 80), Hand of Reckoning's chance to miss was still 7.0%. This gap was covered with [Glyph of Righteous Defense] (removed in Patch 4.0.1).
0100Hotfix (2009-08-08): "Hand of Reckoning is no longer able to be used on targets that are immune to taunt, including players."
0300Patch 3.2.0 (04-Aug-2009): Redesigned. Now does damage only when the target does not currently have the caster targeted, but damage done increased to 50% of attack power, occurring after the taunt effect is applied.