The Virtuoso has the ability to become the most effective barrier source in the entire game. His barrier generation is capable of providing Arcane Warrior-like resistance to the entire team, but this comes at a catch: Proximity and Mana. His mana regeneration rate is directly tied to three factors: proximity of enemies, proximity of allies, and attack damage output via auto attack.
One of the most important things to note about the Virtuoso is that he has a radius ring around him that represents the area of effect of all of his spells. Only you can see this ring, your allies cannot. Various passives will require allies or enemies to be inside this ring to work, and other active abilities like Power Chord will have a "degrading area of effect" within the circumference of this ring.
You start at level 1 with four different "chords": Fire, Cold, Electric, and trigger.
As you level up, you will unlock different "songs" that are used by stringing together a combination of these chords. The starting abilities that you get are Hot Licks (Fire-Fire-Fire: a fire-type attack that deals 400% burn damage per strike), Hometown Anthem (Ice-Ice-Ice: barrier), and Fast Beats (Elec-Elec-Elec: increases movement speed of party by 20%).
As you progress through his trees, you will unlock different abilities such as attack speed modifiers, HEAL ON HIT for the entire party (yes, you should take sing-along, but not it's upgrade; I'll get to that), elemental and spirit resistance, armor buffs, an elemental weapon empowerer spell (See my comments on Mark of the Riff, below), and of course, Power Chord--which can do a ton of damage if utilized with other passives.
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PATH OF DEVELOPMENT
Obviously, everyone will be trying out every kind of build to find what fits for them. The most effective use of this class, however, relies in a hybrid mix of offensive and defensive capabilities.
Here are the abilities I recommend for a hybrid bard (barrier gen with time to cast buffs):
---Beeline for the all three mana regeneration passives. This includes a passive called Feel the Crowd in the mid-left tree that increases mana rate for every ally nearby, as well as Combat Clarity in the right-tree, and another little passive called Opening Set in the right-tree that increases mana per auto attack hit <---emphasis on this last one.
--Next, grab the barrier passives: The knight-protector passive is an absolute must, as is strength of spirits. Both of these will result in stronger, longer-lasting barriers that give you more time to play with buffs. Also grab Play to the Crowd in the left tree to power up your own barrier when allies are nearby.
--Now grab Sing-Along. Nothing makes you a more welcome addition to the team than the capability to let everyone heal for every strike. The rate is 2% heal on hit. DO NOT GRAB THE UPGRADE. The upgrade is supposed to allow you to cast Heal on Kill, in addition to heal on hit, but this ability is broken and does not work. Don't waste the point, until they fix it. Also note, en route to this ability, you will pick up a spell called Battle of the Bands. This increases attack speed. Very awesome spell, because in addition to making your team's reaver happy, you also gain more mana yourself for every auto-attack, because you're attacking more quickly. Avoid the upgrade for Battle of the Bands for the purposes of my build.
--Now spec into the right-tree a little deeper and grab Power Chord. This ability is Zither's nuke and deals big damage to close-proximity enemies with a degrading effect for enemies further out. Grab the upgrade, too. This dispels barriers and venatori mines in addition to doing damage.
More offensive build: The encore passive is practically mandatory for Power Chord synergy. This ability allows you to recast a song with no cost everytime you kill an enemy with a song (Power Chord). You can't keep this up indefinitely, but it definitely helps to stretch your power chords. For the purposes of this build, I also like Power of the Dead and Infernal Music, both of which only continue to strengthen Power Chord.
More defensive build: If you prefer a slightly more defensive build, you can forgo Power Chord's upgrade and just take the Encore passive. Then procede to the left-tree and pick up A Little Knight Music, There Is No Darkness, and Winter Music. While A Little Knight Music's armor value should continue to rise as you level up (at least it does on mine), even at a high level it has limited usefulness next to barrier. The real gems on this side of the tree are There Is No Darkness and Winter Music, both of which apply powerful passive resistances to you and your party.
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COMBAT STRATEGY
When encountering a group of enemies, immediately drain all mana into barrier for the team. With the passives that I've mentioned, your mana will rapidly replenish with proximity and alternating auto-attack, and your barriers should decay slowly enough at this point to where u can begin applying buffs.
Apply buffs. Battle of the Bands is the most useful at this point (+ attack speed), but make sure you're prepared to switch back over to barrier or Sing-Along (heal on hit) when necessary.
After you've applied buffs, queue up barrier again, but begin auto-attacking. It's best to actually wait until barrier is very low on allies to start using it again, the idea is to try and get mana as high as possible to begin Power Chord attack---but don't hesitate to drain all mana for barrier again if needed.
When mana is high enough, throw on just a couple barriers, auto attack, and prepare for power chord. When mana reaches max, and allies are still safe, drain all mana on power chord attacks until all enemies in your radius are dead. Note that Power Chord's upgrade dispels barriers and venatori spellbinder mines, making it a very powerful offensive version of Dispel. Power Chord should be bumped up in slightly higher priority when enemy barriers or mines are present, since dispelling these obviously helps to speed the battle.
Resume auto attack and barrier protocol. Rinse and repeat.
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THINGS TO NOTE
Cool Beats on Zither's right-tree is just not worth the points, at least in my opinion. It looks tempting, but in practice it does damage so slowly and your kit has other important demands for it to be worth the points.
Bolstering Ballad on the left-left-tree is also not worth the points. I've tested this one extensively and concluded that with perpetual barrier it's just doesn't matter. Best to hit the defensive passives above and save the other two points for something better.
Mark of the Riff: I have tested this ability out and have nothing against it personally. However, I don't believe that it is worthwhile to take. This is because in order to obtain this ability, and it's weaken upgrade, you have to sacrifice 3 or even 4 ability points that could otherwise (and should otherwise) be spent in other areas. This ability is a powerful buff, make no mistake, but the fact that it lasts for a set number of hits as opposed to an actual 12 second duration makes it much more difficult to maintain. Furthermore, the sacrifice necessary to obtain this is a poor trade-off. Powerchord is essential for dicey up-close situations and dispelling enemy effects [this is in reference mostly to perilous], and the sacrifice of other defense or barrier-boosting buffs is too high. But, as always, take this guide as just that, a guide. Ultimately, go with what works for you and your team the best.
Never leave home without lyrium potions. These are the single-most important potions you can have for this kit. The mana boost you get is invaluable for keeping up those barriers in a sticky situation. Don't hesitate to use.
Equip those attack rings. Sounds strange on a mostly defensive kit, but mana regen via Opening Set is directly proportional to the amount of damage inflicted on an enemy for every auto attack. Not to mention attack rings will boost Power Chord and stacks with other passives for increased damage. Try to go with two superb attack rings if possible. For amulets I like either cunning/dexterity (more crits = more mana) or constitution. For belts, just go with health. No cooldown amulet required for this kit. Also note that HoK rings are not particularly necessary with sing-along and perpetual barrier gen.
BARRIER RINGS: I have tried to test this out with others in beta, but from everything we could determine, the barrier ring was incompatible with Zither's version of barrier. This may have changed since the beta, but I've seen no confirmation on this. Until it's officially confirmed, I advise against equipping a barrier ring unless you don't have a second attack ring.
Zither does NOT have guardian spirit. He will die shamelessly if you neglect barrier. While you will find that your own personal barrier generates much more than allies' barriers, if you take a certain passive in the left-tree, if you neglect barrier duty on either yourself or your team you will die, or worse: your team will die and you will be soon to follow.
Enemy archers suck. Take cover whenever possible. While it's easy to think you're immortal on this kit, getting reckless leads to stupid mistakes. Faceplanting a hail of enemy arrows will be very taxing on your mana regen and your barrier will falter--particularly on perilous. Remember you can barrier teammates through walls so long as they're in range.
Don't chase after pugs. The single most common mistake I see is bards chasing off after the guy who wants to run ahead and agrro the whole mob. This is bad. Your mana regen will drop considerably without the rest of your allies present, and worse, you have only one teammate for backup. If the pugs want barrier, they will learn to come to you. The true beauty of this kit is that it's supremely good at punishing people who wanna run ahead of the rest of the team. Always stick with the majority, if you can. If an ally leaves your radius, you will take a hit to mana regen, but it's a smaller hit than if two leave your radius when you're chasing after the pug. In the spirit of that.... remember that you can't save everyone. When the going get's tough, wish the fallen luck on their journey and try to protect the person with the greatest damage output.
Lastly, always default to barrier. The buffs and power chord are very nice compliments but the true worth of this class is it's ability to generate barrier constantly. Don't juggle too much at one time. If you're facetanking the red templar commander, best to just stick with barrier. As long as the team doesn't get hurt, they won't really need the buffs.
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RELATIONSHIPS TO OTHER CLASSES
If you have people breaking off in all directions, and you will, use this as a general guide about who to stick with and who to abandon. ...This is obviously an opinion-based rough guide to general pugs... use your own judgement here.
Reavers: Reavers and virtuoso's are best friends. Protect reavers at absolutely all cost. Even if you have to die to do it... not kidding (you have the keeper passive that heals allies' on your own death).
Duelist: While the duelist is survivable in her own right, the addition of barrier and attack speed buffs are just as critical for her--and by extention, for you--as the reaver. I give the reaver higher priority because it is more of a glass cannon, but ideally, the duelist should be almost always within your range.
Keepers: Keepers are your other half: the static cage and power chord combos are invaluable, and with the addition of a strong barrier backup, you two should be stuck at the hip no matter what. (unless it means protecting a reaver or duelist instead).
Archers: Archers will naturally come to you but it is not your job to chase after them. If they leave your side they will die, and rightfully so.
Necromancers: Necros need to learn to come to you, but do try to look after them a bit; their walking bomb damage is nice to have in tight corridors.
Legionnaires: They don't even need you in most cases.
Arcane Warriors: Don't bother.
Elementalist: A good ele can take care of himself; bad ones die.
Katari: Katari's are survivable by themselves with the occassional barrier buffs.
Avaar: Avaar should try to be under your wing at all times, but they tend to run away---FAST; not worth chasing after in most cases.
Templars: Templars are very survivable with minimal babysitting.
Hunters: They can stealth but they should be smart enough to stick to where the barrier is.
Assassins: Assassins ought to be alright on their own with the occassional sing along and barrier boost.
Alchemists: If they're pugging, let them rip and wish them luck.
Hopefully this helps! ZITHER needs to live to play his next show!