Wikia

Thrash
Thrash Melee range Instant cast Requires Cat Form, Bear Form Thrash all nearby enemies, dealing immediate physical damage and periodic bleed damage, and also causes the Weakened Blows effect. Damage varies by shapeshift form.
Usable by
Class	Druid
Properties
School	Physical
Talents
Specialization	Feral, Guardian
Level required	28
Other information
Requirements	Requires Cat Form, Bear Form
Related debuff
Weakened Blows Reduces physical damage dealt by 10%. Duration: 30 seconds

Were you looking for the mob/NPC ability or item ability called Thrash?

Thrash is a druid ability learned at level 28 for druids who have chosen the Feral or Guardian specializations.

Patch changes

Patch 5.0.4 (28 August 2012): Updated
Patch 4.2.0 (28-Jun-2011): Thrash initial base damage has been increased to 1042, up from 339. Initial damage attack power scaling has been decreased to 9.82%, down from 19.2%. Periodic base damage has been increased to 581, up from 189. Periodic damage attack power scaling has been decreased to 1.67%, down from 3.26%. In addition, a bug was corrected where armor decreased the periodic damage done by this ability.
Patch 4.1.0 (26-Apr-2011): Thrash bonus threat has been removed and replaced with increased damage done.
Patch 4.0.6 (8-Feb-2011): The tooltip for Thrash’s bleed effect listed the amount of damage per tick rather than the total damage for the effect. This has been corrected.