Any damage taken by the target will break the effect. This includes direct damage, AoE attacks, damage over time and other effects such as [Hand of Sacrifice]. However, damage from the Censure debuff - caused by [Seal of Truth] - will not break the effect.
Repentance is very useful in both PvE and PvP:
In PvE it can serve as an excellent form of crowd control, with its 1 minute duration and 1 minute cooldown allowing paladins to keep a single target incapacitated indefinitely. It can also be used in emergencies or in order to allow the healer to catch up on damage. Unlike some forms of crowd control it works with many races - Humanoids, Demons, Dragonkins, Giants and Undead - but bear in mind that many mobs (such as Bosses) are immune to effects such as Repentance.
In PvP Repentance is particularly useful. It can be used as a basic stun/interrupt, halting important casts or giving you time to heal. Its 30yd range means it can be useful against ranged types, allowing you to get close to them while also stopping their attacks. It can also be used along with [Hammer of Justice] to allow the paladin to stun/incapacitate two targets at the same time. Since it breaks on damage, Repentance is perhaps most useful for incapacitating a secondary target (or healer) while focusing on another target, or in order to give you a chance to self-heal.
Repentance is the second crowd control ability available to Paladins (after [Hammer of Justice]).
Trivia[]
Repentance is a common Paladin spell in Hearthstone. The flavor text reads: "Repentance often comes in the moment before obliteration. Curious."
Patch changes[]
Patch 10.1.0 (2023-05-02): Duration decreased to 6 seconds in PvP combat (was 8 seconds).
Patch 10.0.7 (2023-03-21): Moved to row 4 (was row 3).
Patch 10.0.0 (2022-10-25): Now a row 3 class talent (was a level 30 talent).
Patch 9.0.1 (2020-10-13): Now available at level 30 (was 45).
Patch 8.1.5 (2019-03-12): Mana cost reduced by 40%.
Patch 7.2.5 (2017-06-13): Repentance no longer deals damage.
Patch 7.0.3 (2016-07-19): Now also deals damage of up to 25% of the target's max health over 1 minute. Now available at level 45 (was 30).
Patch 5.2.0 (2013-03-05): Now has a cast time of 1.7 seconds (was 1.5 seconds).
Hotfix (2012-10-04):Both Repentance and Hammer of Justice can now be applied to the same target.
Patch 5.0.4 (2012-08-28): Now available to all specializations as a level 30 talent (was a tier 5 Retribution talent). Cooldown reduced from 1 min to 15 sec.
Patch 4.2.0 (2011-06-28): Many crowd control abilities no longer cause creatures to attack players when they are cast, including Repentance.
Patch 3.0.3 (2008-11-04): PvP duration reduced to 6 sec.
Patch 3.0.2 (2008-10-14): Mana cost changed to 9% of base, duration increased to 1 minute (10 second PvP duration), now usable against Demons, Dragonkin, Giants, Humanoids and Undead.
Patch 1.9.0 (2006-01-03): Now considered an Incapacitate effect.
Patch 1.6.0 (2005-07-12): Duration changed from 6 seconds to up to 6 seconds.