Many MMORPGs (the most recent one is Age of Conan) have taught players that tanking is something for PvE, and that in PvP a tank needs to spec for damage (turning himself in a gimped DPS with lots more HP) to be effective.
This is NOT the case in Warhammer Online. Sure, a tank can still spec for damage here, but it's, in most cases, not the best thing to do in RvR, unless one likes to go out solo or has plenty other tanks and too few DPSs in his usual group/warband.
This, of course, doesn't mean that offensive tanks are useless or don't have a role. But they are normally less useful to their realm mates as a whole, and are probably better suited for solo play.
Tanks normally have an offensive mastery path (As said above, more useful for soloing, the one on the left of the mastery menu), a defensive mastery path (in the middle) and an hybrid/utility mastery path (on the right).
In my opinion the best way to setup a defensive tank for RvR is with a mix of the defensive and the hybrid mastery paths. The defensive mastery path is a no-brainer, you're a defensive tank, and it gives you the most survivability. The hybrid/utility mastery path gives you that bit of damage you need, and still includes some great abilities to integrate defense, some of the best CC abilities and others (like silences, or disables) that will allow you to protect your mates better and harrass the enemy squishies.
There's no "surefire" way to mix up the two masteries, and favoring the defense one over the hybrid one isn't always the best option. For instance many defensive swordmasters favor Hoeth because of the silence that comes with Whispering wind, while others prefer the shorter but more flexible knockdown coming with Chrashing Wave and end up favoring Vaul, depending if they prefer to defend their realm mates more against melee DPS or casters. The best way to decide what adapts to your playstile the best is to experiment by yourself mixing the two rightmost masteries and see what you prefer.
I would advice NOT to go over the last ability you get (on step 13) on either mastery, since going all the way on a mastery path will gimp the other and will grant you just a rank 4 morale that you don't really need, since the "basic" rank 4 morale all tanks have (Immaculate defense) is perfect for a defensive tank, and the one acquired with the mastery would be redundant.
The Tasks of a Defensive Tank
- Defending your group and realm mates
- Harrassing the enemy healers and DPSs
This task is your secondary task, but it can be performed from the very start, so I'm talking about it first.
Being a tank allows you to get deep into the enemy lines (not too deep, or you will overextend your own healers and die, no matter how many HP and armor you have), and harrass their backline characters, supporting your melee DPSs.
Do you need to spec for damage or deal lot of damage for that? Not necessarily. Just by starting to beat on the face of the enemy healers and DPSs you will distract them from their task, possibly you will force them to move and kite you. This will deal a severe blow to their performance, and ultimately will lead (if your mates are doing their job well) to your side's victory.
When your level grows you will gain some of the most powerful Crowd control abilities of the game (that allow you to immobilize your enemies or root them on the ground for a few seconds, or slow their movement), some disorients (they increase the casting time of the enemy abilities) and some tanks get even some silences (prevent the enemy from casting for a few seconds). It's not random that such extremely useful abilities are almost all concentrated on the defensive and on the hybrid mastery lines. This makes such masteries extremely precious in harrassing healers and DPSs, quite a lot more than pure damage (there are other classes much better than tanks at that).
Remember, though, that this is your secondary task, and as such, it should be performed only when the squishies on your side are safe, or already have plenty tank support.
Defending his realm/group mates the true ability of a tank, and, unfortunately, the most neglected by many.
As I already said many other games got players used to the fact that tanks are just gimped DPSs in PvP, and can be "true tanks" only in PvE, due to the inability (in most games) to automatically force an enemy to turn target on them like a mob does.
This is absolutely untrue, and I dare say that if an higher percentage of tank players actually learned to play their class as real tanks, many of the complaints about the perceived overpowered condition of many DPS classes would cease.
As a defensive tank you are the goalkeeper of your party/warband. If the enemy strikers filter through your lines, your task is to make sure their slam their ugly faces on your shield, and to make sure that the process is the most painful possible for them.
First and foremost, the very basic rule. As a defensive tank, you will carry a shield. Leave the big weapons to the ones that need to compensate. You got a job to do.
The ability to tank in RvR will mature with level (even if collision detection still makes you pretty useful even just as a force of interposition, blocking the path of enemy DPSs to your squishies is still a way to protect them). You will soon earn the following abilities:
Taunt (level 7): while it's not very powerful for tanking, it increases the damage you deal on the taunting enemy. It's even more useful for harrassing enemy squishies (see above), but still, a taunted melee DPS homing on your squishies will still receive more damage (and as such die earlier), and as such it's a good idea to use it if you have AP to spare, expecially if you still don't have other tanking abilities. Moreover, it has the same graphical effect as Challenge (see below), and this might lead the targeted player to think he's actually under the effect of challenge.
Guard (level 10): at level 10 your tanking ability will begin to grow considerably. Guard allows you to halve the damage done to a friendly player by taking it upon yourself instead. It actually works really well while using a shield (again, a defensive tank should almost always use a shield in RvR), because the damage you take from Guard instead of your ally CAN be blocked with the shield, so many times you'll end up halving the damage dealt to your companion and receiving a much lower percentage yourself. This ability gets extremely useful if your situational awareness is well honed. Many tanks are good enough to switch guard dynamically between different party members as they see them in trouble, effectively protecting their whole group with it
It can be useful even while harrassing the enemy DPSs and healers actually (see above): just throw guard on a fellow melee DPS and follow him, assisting him against his targets. While you disrupt the squishie with your disorients and your CC, he'll finish him quickly, and his survivability will be increased by a lot, allowing him to kill more enemies before (if) he goes down.
Hold the Line (level 10): This ability is possibly the most useful ability a tank has in open RvR, and the primary reason why a tank should always carry a shield. It gives you a 45% chance to disrupt any enemy spell incoming to you and to dodge missiles (yes, 45% chance of negating ALL damage from them). In addition to that, it gives the 15% chance to do the same to all your allies (even outside your party) in a very wide arc behind you. On top of that, up to 3 HTL from 3 different tanks can stack on the same person, meaning that 3 tanks doing just that will give a 45% protection to ALL allies behind them. It depletes your AP over time, but this means it doesn't have a cooldown, and you can use it almost continuously as soon as your AP regenerate.
This ability is EXTREMELY useful in open RvR zerg vs zerg combat, expecially in theinitial phase where the zergs are about to collide and ranged DPSs still have no one disrupting their action and as such are the most dangerous. Just a couple (paramount would be 3) tanks using Hold the Line will give a MASSIVE protection to all their mates behind them (and to themselves of course), granting a bonus to survivability so big that it's actually hard to factor.
Too few tanks know and use this ability well, and that's why they are so ineffective. In reality, HTL is a tank's best friend, and the best friend of everyone standing behind him.
Challenge (level 20): with this ability a tank's arsenal is complete. It hits every enemy in a pretty wide arc in front of you, and reduces their damage by 30% against everyone that's not you (this means that a challenged DPS that's attacking the one you're guarding will deal to him just the 20% of his normal damage) for 15 seconds, unless they turn on you and hit you 3 times (oh please do that, lol). learning positioning and situational awareness, in combination with this ability makes a tank invaluable in keeping his realm mates safe. All of them, even the ones not in your group.
In addition to that, a tank gets some of the best Crowd Control abilities in the game. Knockdowns, Roots, Snares, Knockbacks. A melee DPS that cannot move won't be able to reach your squishies. A Caster that's knocked down won't be able to deal damage or heal (not to mention run away from your melee DPS friends), a melee DPS that's knocked away down a cliff will have a long walk to reach your squishies again (unless you knock him directly in bad terrain, like lava, in that case it's almost an instant kill). Those abilities, in combination with the ones I listed above, simply make defensive tanks (the ones that mixed the defensive and the hybrid masteries) a wall of steel that will protect your damage dealers and healers in an extremely effective way.
A special word has to be said about Knights of the Blazing Sun and Chosen. Those two classes are given group-wide auras that enhance your group performance by a LOT. As such, they can easily be defined the most group-friendly tanks (expecially if defensive). If you can twyst such auras by activating a new one after about 5-6 seconds, you'll be able to have (since each of them "lingers" for 12 seconds after it's been replaced) to keep up to three auras active on your whole group at the same time. If twysting every 6 seconds is too taxing for you, even just twysting two auras every 10-12 seconds is plenty to guarantee a new performance. Remember, though, not to let your twysting get in the way or actually tanking. It makes no sense to let a party-mate die not to loose your twysting rhytm, so, if the situation gets dire and fast-paced, you can just activate your most appropriate aura and do your task, resuming your twysting when you can. I repeat: It's better to have just one aura/command active and save your party mate that's being hard pressed, than having three and let him die.
As to what aura/commands to use, normally the most useful for defensive tanks are the ones in the defensive/hybrid trees. They are very situational, so you'll have to use your judgement on which ones to use at any time. For instance, if your party is being maimed by caster, using the resistance aura (gather your resolve for KOTBS and Discordant Instability for Chosen) will help more than the others. The best aura tanks can dynamically change the auras in their twyst as the situation on the battlefield changes.
As a word of waning, remember that using any ability (twysting included) will interrupt your Hold the Line, so, if you're holding the line, it's better to avoid twysting or to resume yoru HTL as soon as you twysted to a new aura.
Finally, the "harrassing the DPS" play can be easily done in reverse. harrassing the ones that filter behind your lines to kill your squishies instead of charging forward yourself.
There's nothing like imagining the frustrated face of that witch elf when she appears behind one of my bright wizards only to find herself knocked into the ground instantly and then challenged, before the bright wizard obliterates her, when she's almost completely unable to hurt him in any tangible way (because I'm probably also guarding him). Two witch elves? Twice the fun. Most of a defensive tank's abilities are AOE, and guard works on both. More often than not, if the tank is reactive, it ends with the two DPSs down while the squishy is walking away pretty much safe.
Remember that as an offensive tank you can deal quite a bit of damage, but as a defensive tank, if you allow even just one ranged DPS to survive double the time, he'll be MUCH more useful, damage-wise, than you, while specced damage, would ever be. Just think about what your faction gains when you manage to increase the survivability of more than one.
Defensive tanks play an absolutely vital role in sieges, and sieges are a big part of the game.
When you're defending, your role is pretty elementary, but vital. Defensive tanks can form an almost impassable wall of shields on top of the ramp to the keep lord room, but not only: Before then, they can still sortie the main door of the keep and knockdown/knockback/harrass the enemy outside, slowing their damage on the doors and giving your allies time to come and defend. Your survivability ensures that you'll have plenty time to get out, do what you need, and then get back in safe.
When you're part of the wall of shield make sure there are healers in your group, because you will be exposed to a rather nasty barrage of spells. If there's none, ask the warband leader to switch a shield to your group, or alternatively to switch you to a group with an healer. Even if you're big, buff and fully geared, you won't survive the barrage if you aren't properly healed.
As an apparent paradox, defensive tanks play a vital role even in attacking keeps. Not only they can stay on the door without too much fear of the oil and of enemy spells (if well healed), but they are basically the only ones that can run up that cursed ramp to the lord room, fire all their defensive abilities, knockdown/back the tank wall, get behind them, cross the lord room, get on the second ramp and start maiming/disabling/harrassing the healers/ranged DPSs perched on it.
This will give your squishier allies a chance to get to the lord room themselves and to kill the tank wall (that if you did your duty will stop receiving as much heals) without being maimed too much by the casters you are harrassing.
Your survivability gives you a decent chance of surviving it to the end. Most other classes (including offensive tanks) would be obliteraded while they're still dealing with the shield wall on top of the first ramp.
When you can walk into stacking rain of fires and pits of shadows receiving ticks of 200 damage (and you have 9000+ HP), you really begin to learn the true power of a defensive tank.
A dead enemy is a dead enemy. It doesn't matter if you're the one that killed him, or he's been killed by the squishy glass cannon that you've allowed to survive. On top of that, that squishy glass cannon probably killed a few others in the process. If that happened, you've done your job.