Warlocks are very good at leveling, having good self heal capabilities and tools to survive. They level at an average pace early game but at later levels you become an unstoppable killing machine, able to kill multiple mobs with ease and becoming one of the strongest classes at leveling.

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The general strategy is to level as Affliction, you will rely on Damage over Time (DoT) spells to do the bulk of the damage, you’ll be building your talent tree with the goal of fighting multiple mobs at once. You’ll get your first major power spike at level 30 with Siphon Life, giving you a decent self healing. Once you get access to Haunt at level 60, you will have another huge power spike, with even more self healing. At level 71 you have another power spike where you get access to Soul Link, reducing the damage you take. Having more self healing and taking less damage means that you can take bigger risks and pull more mobs simultaneously.

Soul Shard Management

At level 16 you get access to the improved drain soul talent, this talent gives you a decent chunk of mana every time you kill a mob while using drain soul.

This following macro will help you with inventory management, destroying all the soul shards in your bags and leaving you with 5 shards:
/run i="Soul Shard"d=GetItemCount(i)-5 for x=0,4 do for y=1,GetContainerNumSlots(x) do if (d>0) then l=GetContainerItemLink(x,y) if l and GetItemInfo(l)==i then PickupContainerItem(x,y) DeleteCursorItem() d=d-1 end end end end


For leveling you’ll be using 2 out of these 3 glyphs:
Glyph of Siphon Life
Glyph of Curse of Agony
Glyph of Quick Decay

You’ll use the Curse of Agony glyph early on since you have little to no haste gear before level 70. Once you acquire a decent amount of haste (around 5%) you swap out the agony glyph for quick decay. Glyph of Siphon Life is the most useful one, since it allows you to play very aggressive, you have to aim at dotting as many mobs as you can. As you progress through the levels, you’ll get better self healing tools that allows you to keep pulling mobs with minimal downtime.

Levels 1-30

From levels 1-30 you have to prioritize mana efficiency so you don’t have to stop to eat and drink so your priority should be:
Corruption > Curse of Agony > Wand. If you don’t have a wand equipped, you’ll have to fill the gaps in your rotation with Shadow Bolt or Immolate, Shadow Bolt tends to be more mana efficient than Immolate at lower levels. Also, if the target is not dying within the full duration of Immolate, it’s not worth casting it.

Levels 30-50

At level 30 you get access to Siphon Life and Drain Life rank 3. Drain Life rank 3 is quite strong and at that point casting drain life is more efficient than using a wand. You will get a strong boost in your ability to self heal at 30, at this stage you should be able to pull and dot multiple mobs without a lot of issues. You can do big aoe pulls with Hellfire, and if you chose to do so, make sure that all mobs you’re pulling have a Corruption DoT on them, so you can heal yourself back up. It’s recommended to have your Voidwalker‘s Sacrifice + Healthstone ready before going for big pulls.
Your priority should be:
Corruption > Curse of Agony > Drain Life – Try to cast Corruption + Agony on as many mobs as possible.

Levels 50-60

At level 50 you get access to Unstable Affliction. This spell does very decent damage but has a cast time. You should still prioritize Corruption and Agony, since you can cast those spells on the move. Having an extra affliction damage over time spell means that you get extra healing from your Drain Life via the Soul Siphon talent.

Corruption > Agony > Unstable Affliction > Drain Life

At level 61, your priority is to get 2/2 Improved Felhunter so your Felhunter can serve as a mana battery and 2/2 fel synergy, so you don't have to re-summon your Felhunter very often.
At 71 you get access to both Haunt and Soul Link, you should visit your class trainer and respec at level 71. At this stage, you won't rely on your Felhunter to serve as a mana battery and you can play with whatever pet suits your playstyle, soul link makes you very tanky and you'll get most of your mana back with life tap, mana won't be an issue so you won't need 2 points in improved drain soul. After you get Soul Link and 3/3 Demonic Aegis from the Demonology tree at level 74, all the remaining talents are quite weak, so you can put them on whatever fits your playstyle.
Dual Spec
If you decide to spend 1000g on dual spec, you could have your second spec as a hybrid Demonology/Affliction build specialized in killing elite mobs. The key talent points of this build is Siphon Life and Improved Drain Soul from the affliction tree for threat reduction and self healing and Improved Health Funnel and Demonic Resilience from the Demonology tree to make your felguard take little to no damage from mobs. Most elite quests up to level 70 are soloable as an affliction warlock, that changes in Northrend where you have quite a lot of difficult elite quests, so having this secondary spec as an option can be quite handy.