Patch 5.1 introduced pets to the vanilla-era raids. Patch 4.3 introduced transmogrification, which brought old raids back. Before that, some players would solo or duo old raids just for the challenge giggles. Every new expansion has raised the level cap and opened another expansion to soloing.
Whether you're soloing for a look, for improved familiarity with your spell toolbox, or for bragging rights, warlock is definitely a great class to jump into soloing old raids. At level 90, even before you're decked out in tier 14, most of the Wrath of the Lich King raids are fairly easy. But let's start with the basics -- speccing and playstyle.
I started soloing old instances as a way to learn how to tank on my bear, so I started with the well-done druid soloing guide at MMO-Champion's forums. While I skip over the speccing advice now, the guide is awesome for judging difficulty and as a cheatsheet for major boss mechanics.
For making the transition to warlocks, there is a less-organized but still useful soloing thread at MMO-Champion. My current warlock source of soloing strategies for Wrath of the Lich King raids is Ysen via his solo videos. As of writing, Ysen has videos go as tier-high as Morchok and Yor'sahj, which is not too far behind the Madness of Deathwing solo by the more famous death knight Mionelol.
Previously, the bulky demonology was the king of soloing, unless you went affliction for the super-heals. At 90, however, the rehaul of the warlock class has left all three specs with strong soloing potential. As with my experience on the bear, tier set bonuses can sometimes help you out -- specifically 2-piece tier 6 (Malefic) gets used quite a lot by some players.
While warriors may talk of talents more than spec, I feel like the warlocks will depend on spec or playstyle winthin spec to choose their talents. Affliction does well soloing with Supremacy while destruction does fine with Sacrifice. The one glyph I like universally is Glyph of Healthstone, which gives the old 100% boost to the healthstone, but in Mists of Pandaria changes the healthstone pop from an instant heal to a HoT. Dark Regeneration is also a great talent to take with or without a pet present, though it's equally as good a pairing to Soul Link as Soul Leech is.
Affliction has lost some of its strength in self-healing and maintained mobile DPS, but it still rocks both categories comparatively, even without boosts like Kil'jaeden's Cunning. The self-healing has shifted from purely through DoT ticks to mostly filler-based or cooldowns. Since affliction specializes in DoTs, the playstyle falls into a rhythmic sea of rolling damage through multidotting.
A strong affliction player is effective at spreading DoTs, whether through manual casting, assisted multidotting, or true AoE. In solo activities, good multidotters can chain-pull with ease, knowing how many mobs they can keep going while still moving. The devs delivered well on the drain-twisting dislike: with a decent amount of haste, Nightfall procs plus smart Drain Soul deaths are enough to stave off shard-starvation.
For soloing, the affliction-voidlord pairing is nigh unstoppable and often Ysen's choice. The voidlord is robust anyhow, and while he doesn't have a great AoE threat-snap, he responds quickly to taunting targets off you now. Additionally, while you're leeching him health, Soulburn: Health Funnel will not only instantly heal a chunk of his health, but reduce his damage taken temporarily.
Edit: Soul Link replaces Health Funnel. Duh, Megan! Thanks ColterHuffman for pointing that out!
Demonology's strengths lie in on-demand burst, AoE, and pet damage. Affliction's burst dies if DoTs are ineligible (immunity, deflection, unwanted deaths from mind-control effects, etc.) and destruction's burst requires significant ramp-up, especially off cold starts. With only the Corruption/Doom combo rolling, demonology is a master with sensitive timing of its damage output.
I haven't gotten a week yet to try out demonology fully in solo mode -- this week was delicious fun with destruction -- but I've heard of some soloers still preferring it.
Demonology has two modes for soloing: normal Metamorphosis (henceforth Meta) use and Dark Apotheosis (henceforth DA). Considering you will do more damage than your pet and your blueberry and felstormers will often tank better than you will, I lean on going the normal Meta route.
This is not to say you can't bust out the tanky DA form -- do it if you want to, but it probably requires some creative stance-dancing to get the necessary DPS up to beat encounters. Going for the mitigation from Fury Ward saps all your demonic fury. Early DA use happened in beta when warlocks would go beat up Morchok for fun and profit easily with the tankish 6% crit immunity. Now that that's gone, Fury Ward or other mitigation abilities like Unending Resolve or Dark Bargain are the only things standing between a DA lock and squish.
You might think the long cast times would put a damper on destruction, but it actually plays delightfully fast-paced, especially with AoE and cleave situations.
I partially credit the new Rain of Fire, whichas of patch 5.1 doesn't even need Immolate anymore to generate burning embers for you. However, this means that destruction favors pulling a boss directly off a four-ember fresh trash kill, and can require some timing to get that. I could easily kill Patchwerk sans-pet as destruction off the trash pull, but the cold-start kill was a little hairier.
The best change of patch 5.1 wasn't Rain of Fire generating embers, however, but Kil'jaeden's Cunning swapping out the cast time penalty for an increased movement penalty. Kil'jaeden's is favored by the channeling aff'locks, too, but there was simply nothing more depressing than trying to move while casting a non-Backdrafted Chaos Bolt with Heigan's Spell Disruption still up. I had eventually settled for actually using the cooldown during the dance phase rather than sticking to the passive all the time.
Ember Taps are simply amazing as destruction, and are the reason I can kill Patchwerk truly solo -- no pet involved. I pop an Unending Resolve and maybe a little Dark Bargain at the end when Patchwerk hits his frenzy, but the bloated abomination surely falls to my self-healing superiority.
For soloing trivial tiers of content such as Vanilla (tier 1 or tier 2) or Burning Crusade (tiers 4-6), destruction is my favorite. In Naxxramas (tier 7) I still used destruction, but beyond that I've gone back to the affliction-voidlord pair. The voidlord is simply a better demonic tank than I am.
Once you get into Ulduar (tier 8), some encounters like Thorim require creative pulling and kiting rather than straight gearpower. Likewise, I had to use my pet to get snobolds off me on Northrend Beasts (Trial of the Crusader / tier 9), and Ysen uses the snot out of his Demonic Gateway and Circle on his Dragon Soul (tier 13) adventures.
Since the companion pets have been introduced, I'll start my warlock perusal of soloing raids back at the Vanilla raids of Molten Core, Blackwing Lair, and Ahn'Qiraj. Naxxramas is a post on its own, due to its size. Not only are there pets now, but old mounts and plenty of transmog gear to stuff somewhere "just in case I need it later."
Raid soloing can be a lot of fun -- besides, it's also a "brawling" experience without needing an exorbitant amount of gold to gain access. Simply walk inside.
Blood Pact is a weekly column detailing DOTs, demons and all the dastardly deeds done by warlocks. We'll coach you in the fine art of staying alive, help pick the best target for Dark Intent, and steer you through tier 13 set bonuses.