Info

Tuska's Wrath is a basic Constitution ability that can be unlocked for 4,000 Reward Currency from the Anima Islands Distraction & Diversion, which became available following the fall of Tuska at the Tuska Comes world event. It requires level 50 Constitution to use. The ability deals damage equal to 10,000% of (or 100×) the player's Slayer level (including any stat changes) against their current Slayer assignment. It will use this calculation even if that number would be less than 110% weapon damage. 

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When used on a monster that is not currently assigned or on a monster that is unaffected, the ability will deal 30–110% ability damage and administer a 15 second cooldown. Every boss except The Magister and Legiones is immune to the effects of Tuska's Wrath when on a Slayer task. 

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The damage cap of this ability is 15,000 life points. Against a Slayer target, the attack will always be treated as critical, severely weakening or killing the Slayer target when used against them. 

 

https://runescape.wiki/w/Onslaught