Actually, it will kill most NPCs outright in one shot, then reanimate them to be your thrall. The strongest enemies won't die right off the bat, but if you knock their health down to around half or slightly higher, it will. Even Drauger Death Overlords of Ultimate Deathly Death fall victim to it.

It'll even one shot regular dragons right out of the sky.

All it does ( in terms of damage) is 300. Nothing to be sniffed at but I have found if the 300 isnt what kills the enemy then no thrall for you.

I've never one shotted a Draugr Deathlord with Soul Tear.

I never said it could. But, if you knock their health down to about half it'll kill them then resurrect them to fight for. Very handy at higher levels when facing a bunch of undead.

Normal NPC's, like citizens, guards imperial/stormcloak soldiers, bandits and the like will kill over in one shot. Even some bosses die in one shot to it. I ended the fight with Vyrthur on the balcony that way before he even had a chance. I've knocked normal dragons (the kind that just says "Dragon") out of the sky with it in a single shot, but they don't show up often.

is it a one-target shout or is it 'everything in it's path gets cut down" shout?

One target shout. And no, it won't resurrect dead dragons to fight for you. Powerful souls and all that jazz...

Is the effect instant?for example,you shout,and when the shout hits the enemy is less 300 hp in an instant or it takes time to deal?for example,10 damage p second?

Considering you svck the soul out...
What I think would be cool, is if the last expansion had a shout that, for the cost of one Dragon Soul (and therefore the cooldown was 0 so it was instant) you could resurrect any Vanilla Dragon. The Dawnguard and Dragonborn Dragons would take two souls since they're so ridiculously OP.
Then the Dragon is Dead Thrall'd.

I actually have a question about the Bend Will Shout. I've heard that besides calming your enemies it also makes them fight for you for a short time. Can anyone who has the DLC please tell me if that is true or not?

Yes it is true.

@Smas Awesome thanks for confirming that for me! I really can't wait to use it now

I dont really bother with it personally, It's not exactly the quickest shout to recharge.

Mix "Soul Tear" with a "Marked for Death"... Even Draugr Deathlord Overlords turn into mewling kittens ripe for slaughter. Toss in a "Dragon Aspect" before hand to expedite this already efficient method of slaughter.

BTW: Seeing as how long Dragons tend to stay in flight, using this three-shout combo on Legendary Dragons really makes those fights less... tense.

Isn't Marked For Death bugged?

Supposedly. Personally I like the Drain Vitality and Soul Tear Combo.

Not to my knowledge. Marked for Death has always worked just fine for me.

But I do know that "Drain Vitality" is bugged. It has the habit of copying the "Marked for Death" words. I have an Assassin who has 3 separate Drain Vitality shouts...

Two of them stole Marked for Death's words, the other one is the actual Drain Vitality.

Bend will can be useful for stealth for when you are detected in an in closed space since by the time it wears off you can be a long way away from the danger, it is also good to stop a battle for a short time if your health is getting low

Drain vitality and marked for death are bugged in terms of learning the words. You have to either learn them in specific order or delete Dawnguard, learn marked for death then proceed with Dawnguard.
Marked for death is also bugged as the actor says Lun Lun Aus instead of Kril Lun Aus.

Yeah I have Marked for Death and Drain Vitality as the same words.. but then another Drain Vitality shout with different words that I'm somehow missing.

No, bugged as in it's extremely overpowered compared to what it was supposed (says it is) to be.