Nightblade is a very good choice for both great damage and great survival/mobility in PvE. Nightblades can use many attacks from 20 meters, just like bards, so these two roles fit together pretty well. But there are also several hardhitting melee attacks for Nightblades so the normal course of action is weaken them with combopoint-generating 20-meter-attacks and moving into melee combat while you do so. Stealth allows for various powerful openers, most from up to 20 meters away.
I like the medium- to meleerange style of Nightblades and when I once respecced into Ranger I also found that Nightblade damageoutput is significantly better, dunno why that is. I used all the ranged attacks a range can do plus wolf pet but still it was awfully long until an enemy went down.
Nightblades also feature powerful weapon enchantments (fire-based) and a pretty powerful crowd control that lets you put an enemy out of action for up to 45 seconds. This is real fun since you will often meet enemies that are too close to each other.
The bard is a good fit for Nightblades because of his early-on buffs, with only 6 points you have a damage-motif (spell and attack power bonus), a survival-anthem and a survival-fanfare (endurance and armor, respectively). Also his life-based ranged finisher comes in handy if your fire-based Nightblade ranged finisher for some reason seems inadequate, e. g. wenn your enemy is highly fire resistant. He will also make you pretty wanted in groups.
The third soul is up to you, really. I went ranger for the tank pet only, I put no points in that class yet. It's nice to have a little meat shield even if it doesn't last long but it's added DPS as well as protection so... fine. Just don't count on it in PvP.
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