Class Talents
Generally, your goal in the Class Tree is to reach the capstones while picking up fight/dungeon-dependent utilities. This means things like extended siglis for longer silences, range on interrupt, purge, stun from Chaos Nova, and the group defensive Darkness. These talents can and should be talented if they suit the content.
The capstones are of varying value. Collective Anguish is pure damage, albeit a significant amount of it. The Hunt is a large cooldown that also grants 50% Leech against your main target of the spell for 30 seconds. The improved Hunt node should always be Unnatural Malice for a marginal damage increase.
Quickened Sigils is a utility option linked mostly to empower your Sigil of Silence. The talent loses value in raid encounters but is very strong in Mythic+. Starting in 10.1 it is also a viable choice to run Elysian Decree due to the synergy with the new tier set. If you choose to do so Quickened Sigils becomes an almost mandatory talent to take.
Beyond, there are only two talents that are borderline mandatory to take. Rush of Chaos makes Metamorphosis a 3-minute Cooldown rather than a 4-minute cooldown. Demonic grants 8 seconds of Metamorphosis when you cast Fel Devastation. Both of these add significant defensive tools to your kit.
Talent Builds
Single Target
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Your goal for Single Target will be to go down the left side of the talent tree and pick Soulcrush and Soul Carver. On the right side of the class-specific talent tree, you want to pick Fodder to the Flame and Down in Flames.
This build involves stacking up Frailty stacks with Soul Cleave or Spirit Bomb during Fiery Brand and generally making the most of the damage boost from Fiery Demise.
Extended Spikes, Down in Flames, and Darkglare Boon all help you maintain some mitigation up at all times through a boss fight and provide you with significant defensive and offensive value.
Cleave
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For Cleave, you basically play the same build as in Single Target. However, you talent into Fallout and move Cycle of Binding to Burning Alive so that you can spread your Fiery Brands to more than one target.
This adds significant Cleave damage since Fiery Demise will apply to any target affected by Fiery Brand. In a Cleave or Multi-target scenario, you will replace Soul Cleave as your primary spender and use Spirit Bomb to spend your souls, even outside Fiery Brand.
Mythic+ Defensive
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This is a very easy-to-play, defensively-oriented talent build. For most mythic+ key levels and dungeons, the cleave build with minor changes can significantly increase your damage and even provide more defensive value. However, this build is a good starting point when you intend to learn how to play Vengeance and how to handle all mechanics in the dungeons.
This is a defensively-oriented talent build. For most mythic+ key levels and dungeons, the cleave build can significantly increase your damage.
The defensive build opts to take Feed the Demon for more uptime of Demon Spikes. This will provide more consistent defensive output since the delayed spread of Fiery Brand can sometimes be too slow to maintain enough mitigation in larger pulls. This build also opts into Painbringer for an extra layer of damage reduction and Last Resort for a cheat death on an 8-minute cooldown.