LoS: The basics
Although warlocks (along with many other casters) have methods for kiting, escaping, or disabling their foes, the key element to any PvP battle is always to know where your lines of sight are. Line of sight (LoS) is a huge defensive and offensive characteristic of all PvP engagements and will be one of the biggest assets in your survival. Getting caught out in wide open terrain is something that you want to avoid every chance that you can. While it isn't always possible to have a handy pillar, bridge, or wall around to protect yourself with, you should attempt to have something you can use at any given time.
Line of sight is such a big deal due to the many uses that it can hold for you. First and foremost, it can be used as a means of interrupting spells -- again, either defensively or offensively. Allowing a caster of any variety the ability to unleash his spells into you without harassment is a quick train ride to tanking the floor, and that's a job for mages, not warlocks.
Never be afraid to run out of LoS from an enemy caster right from the start of a battle. This will allow you to have time to plan and prepare for the coming battle or allow you to escape a harrowing situation. Once you have run out of LoS, your enemy is forced to chase you if he wishes to continue to pelt you with his barrage of attacks; more often than not, this is something he won't be keen on doing, and for good reason.
Far more than just a defensive tool to keep enemy casters from tossing spells at you, LoS can prevent an enemy healer from landing critical casts on teammates that would otherwise save them. When attempting to push for a kill, one of the biggest tactical moves you can make is to force either your enemy or his healer out of LoS. Doing this will prevent healing entirely and allow you more freedom to take down the enemy.
Forcing a chase is one method of doing this; using Fear, Howl of Terror, or Death Coil is another. While our fear/horrify effects have the potential to push players out of LoS from each other, this is not a reliable method of breaking LoS. Using the Glyph of Fear in a PvP situation is very much a trade-off in this regard. Sometimes, a fleeing enemy is a good thing -- it can push him out of range or out of sight of teammates and leave him vulnerable; other times, it will force you to put yourself out of range or out of LoS of your teammates, or the player won't go in the direction that you wanted and he'll end up in a better defensive position then when he started.
No matter what the case and no matter what your choice in Fear, having a method of controlling LoS is a huge benefit in PvP, and you want to have this any chance you can get. A good pillar hump can save your life or score a kill when it's needed most.
LoS: When it fails
Line of sight is a great defensive tool for blocking enemy casts, but it can also be used by enemy players to block your casts. For any warlock but especially destruction and demonology, this can be the most troublesome experience ever when a player starts pillar humping against you.
While doing this blocks most direct casts against a player, this is actually a situation that's highly beneficial to warlocks. Although these two spells are not excessively powerful in terms of damage, all warlocks have two instant-cast DoTs: Corruption and Bane of Agony/Doom. When we're chasing a player, neither of these spells on their own is going to cause much of a threat to them -- but it's a game of outlasting the other player. Using these spells, you can whittle away at his health or drain his team's mana by causing a healer to be forced to constantly spam dispel.
It takes time ... but all you have is time. One of the main rules of PvP: Never get cocky, never get hasty. Do not overextend yourself, do not get overeager, and always be prepared to wait it out. This is a game that warlocks play best at; you can outlast your enemies.
You may be thinking that casting all of those Corruptions and Banes is going to drain your mana too -- and drain it faster than the healer's. This is true, but you still have the advantage. Warlocks have near-infinite mana due to Life Tap, and while dispels in PvP are a major issue right now, you can still easily win out the slow game. Why? Because pillar humping and LoS do nothing to our biggest ability.
Channelled spells are not impacted by LoS. Prior to its removal, this put warlocks at a huge advantage due to Drain Mana, but even Drain Life is a spectacular ability. Once you get your target in LoS for even a split second (which will happen), you can start to channel Drain Life. From there, your target can LoS you until the cows come home; it won't make a bit of difference. Channeled spells will only break via range or interrupting the spell itself; running out of sight of the caster does nothing to prevent it.
This is what gives warlocks their edge. A player may try to constantly LoS you in order to gimp your damage, but it doesn't matter because you can merely whittle his health and mana away with Corruption and Banes while keeping yourself relatively full on both using Life Tap and Drain Life. It won't be quick, but any player who thinks he can outlast a warlock is kidding himself.
Further, a player low on health that thinks pillar dodging is going to buy him the time they need for a healer or teammate to arrive has the way wrong idea. Drain Soul won't break from LoS either, and its damage against a low-health player is devastating.
Never forget that you can out-abuse LoS far better than anyone else -- and let them know it.
LoS won't always be there to save you, nor will it always be enough. Sometimes, you just need to get away, whether it be from a melee player who's chewing at your face or just a battle gone wrong. In many of these situations, you won't always have a simple out, but warlocks are just as tricky as the demons they enslave, with plenty of tools to get them out of a bad way.
Demonic teleport is your first line of defense against being caught in a bad row. Always have one down and ready for use, and always be aware of the position that teleporting will put you in. Many of times I've seen a wayward 'lock who's used his teleport, only to put himself in far more danger than he started in. Don't let this happen to you. Situational awareness is key to victory.
One of the best locations for dropping a portal is near some environmental advantage. Slightly up a hill, on a roof top, just on the edge of a pillar or bridge so you can dart for cover once you arrive -- these are the places that you want to look for when placing a teleport.
Also, consider the dual nature of teleporting. While an amazing defensive tactic, demonic teleport isn't purely a defensive spell. Equally as frustrating as seeing a warlock teleport right into the welcoming arms of a hungry rogue and/or warrior is watching a warlock chase a running target ... right ... past ... his ... circle. If a player is attempting to use your escape route, show him why it's yours and deny him. Immediately port ahead of him and start casting. If he is low, then outright kill him; if he is just trying to flee from overwhelming odds, then disable him with a Fear.
A hallmark of warlocks is that they aren't a defensive class overall; they're offensively based. A mage might hide behind his pathetic barriers and his sissy snares, but a warlock kills to sustain himself. We bleed our enemies and relish in it; use this to your advantage.
Healthstones are an amazing tool that you should have on you at all times. I don't know any other way to say this: If you don't have a Healthstone in your inventory and it isn't on cooldown, then you're doing it wrong. With or without the application of Soulburn, a Healthstone is a major chunk of your health restored instantly. Have it, and don't be afraid to use it.
Shadow Ward is only useful against shadow damage, but it's far more plentiful in PvP than players realize. Death knights, other warlocks and priests all use shadow damage. Random hunters who feel like using Black Arrow on you also deal shadow damage -- but if this happens, you're probably just going to lose because you'll be too busy laughing at the hunter to respond. In any event, if you're being hit by any form of shadow damage, use Shadow Ward! Any reduced damage is better than ... well, everything!
Curses -- use them! Seriously, Curse of Tongues, Curse of Weakness ... all of these are simply amazing. You can cripple both enemy casters and melee rather significantly using these abilities. Curse of the Elements is great when going in for a kill or increasing pressure, but you need to worry about staying alive in order to do either of these things. Be smart and recognize when it is better to reduce a player's damage output instead of merely increasing the damage they take. Having a warrior do 10% less damage is a lot more beneficial than doing 8% more to him.
Oh, our demons, how ... curious they are. As a pet class, we are rather unique in many ways. Not only do warlocks rely on our pets for large amounts of damage -- have you seen the new Shadow Bite?! -- but it is also our utility and a huge portion of our defense.
It isn't uncommon to see players attempting to focus fire on your pet in order to deny you the safety of Soul Link. As much as you need to keep yourself alive, you need to keep your pet alive as well. The Fel Synergy talent is very important for this, and I would highly suggest picking it up in your PvP spec if you can spare the points for it.
Although your pet is a huge portion of your damage, the reality is that you need to play defensively with it. Although warlocks are offensive by nature, hit-and-run tactics usually work best at the onset of a brawl. You want to hold your pet back and primarily use it for utility purposes, such as Spell Lock or Axe Toss, instead of blindly throwing it into the fray. Pets are very squishy targets -- and so are you, if yours dies.
In these situations, the Imp is more so allowed a larger range of attacking due to its having a ranged attack. Other pets should be held back until you can either pin a player in a situation that they cannot gib your slave or you are going for a kill.
Important note: If a combat rogue or warrior gets on top of you and your pet, immediately send your pet away, even if you are not able to escape. In such cases, the rogue is likely to Blade Flurry and the warrior to Cleave, which is going to destroy your pet faster than you can blink. You must get your pet out of danger quickly to avoid this happening. If your pet dies, you start taking more damage; you do not want this to happen!
Know your armors
While it was slightly more controversial the last time we breached this topic, PvP armor follows an entirely different set of rules. Demon Armor is far more amazing then Fel Armor in a variety of situations, although it really shines when you have a healer around to help you.
If you do have a healer present, there is virtually no reason at all to not be using Demon Armor. The increase to healing effects is simply amazing; far more than that, it is a life-saver. Further, the increase in armor does make a noticeable difference. It isn't going to allow you to go toe-to-toe with melee players, but it is going to reduce what damage you do take.
The largest flaw with Fel Armor is that it is only going to heal you off of direct damage spells, which can be few and far between in a PvP setting. You won't often have the chance to stand there and spam your nukes at players; instead, you'll be doing a lot more multi-DoTing and running around. This cripples the life return from Fel Armor rather significantly, without even going into the factor of the reduced damage you'll be doing to players.
You lose out on some damage in using Demon Armor, but the survivability gain is more than worth it. PvP is a game purely based upon the concept of survival; you must do whatever you can in order to live.
Stay aware of what's around you
There is far more to cover than I could ever hope to write, but these are the key features that you need to pay attention to. Remember, always be aware, know your LoS, know your escape points, and know that you are able to outlast nearly anyone. Even as destruction, you won't be scoring any quick kills against most players, so play to your strengths, which is wearing down the enemy a little at a time.
Read Blood Pact: The fine art of staying alive, part 1 (PvE)
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, advise you on tip-top trinkets and steer you through encounters such as Blackwing Descent and The Bastion of Twilight.