Well lets see...
The trick is getting to 20. That's the hard part- but if you go all in for shadow rush it's more like the trick is getting to 12. I swear if I make 20, I can make 30, and then it's just patience and itemization.
Now'd be a pertinent time to say I've not had such hot luck with my favored class since b44 or so. So, take your grain of salt and other vets feel free to contradict.
General Notes: Doomed are *FANTASTIC* at slugging it out. They are miserable at spike damage. Their biggest player mistake weakness is picking fights unprepared (deflection shield down/ low hate for the most part). Their second biggest weakness is not hitting your enemy in his weakness. Ways that make 'em better at slugging it out work great (AM and AM shield + deflection combos, armor + deflection, creeping dark LoS control, all amplify eachother). Finding out how to do spike damage (detection, preemptive creeping dark, preemptive unnatural force, LoS target, agony, focus shadows, wail on 'em for crits <- idealized play). Know thy enemy, and hit him in the jewels: LoS the ranged. Gesture the non stun immune. Knockback yoyo the melee. Madness + agony just about everything.
Force of will tree:
So willful strikes a handy little one point wonder, but it's scaling isn't fantastic. I tend to leave at one until late game- and use it all the damn time too
Deflection: miserable scaling. Fantastic one pointer. Please note that even when deflection shield is emptied, it tries to regen 2 or so 'shield' a turn which is effectively health until you're knocking single digit HP's. Or, in other words- it's two hp regen per turn plus an ablative buffer. 2 hp/turn regen is nice.
Blast: Oh... it's lovely. But I beeline the next skill for its ridiculousness. One point provides a lovely little 3 turn daze breather- if you're not floating agony or hateful whispers already. See how those cause issues?
Unnatural Force: Okay, so at one point it sucks. At three points its good. At five points it's like ~200 damage 3 knockback (random direction) 30% chance of second strike, at 5 points AND a cat it's ~240 damage 40% second strike, which just grinds solo enemies (see: bosses) pretty nicely.
Creeping Dark/ Darkvision are hardly useful at 1/1, begin to be useful in corridors at 3/3, and are fantastic at 5/5, especially in corridors, and especially if you're swinging around movement infusions too just in case of retarded darkness spreads on open ground.
Please note that the +crit damage from force of will and +damage % in darkness tree combine multiplicatively.
Dark torrent: well, here's the gap filler for reproach / willful strike / agony spam, it'll make a full rotation. ~20% chance (CAN WE GET THIS MOVED TO MINDPOWER BASED?) to blind and make target(s?) stop targetting you: aka go hit shadows again. Or something like that, I've never really noticed and doubt it works. However nowadays it does leave some nice creeping darkness trails pretty often, which can act as carpets with the +move speed on darkvision for gesture users.
Dark Tendril: once upon a time this was semi useful for LoS abuse. Now, it's useful vs bosses, maybe, or the rare knockback immunes (wyrms!). I don't recommend it really at all anymore, it's mediocre in comparison to throwing more points at madness or unnatural force, or even dropping a cat point and getting panic out of fears. I'd rather have 1/1/1/1 in fears than five in tendril.
So, step 1, get some shadows. Step 2, 1 point in shadow warriors for fade. Step 3, get 4 points in shadow mages and they start ressing themselves: NOW you profit.
Note that focus shadows has a percentage chance to trigger shadows using blindside on target- IF the shadows can see the target. HOWEVER, shadows are guaranteed to teleport to and protect a targeted ally instead of 'checking' for blindside, and you can focus shadows a shadow that decided to actually go smack the right enemy...
Shadows suck in vaults. You'll waste plenty of hate just tryign to get them to follow you back into the no teleport vaults thanks to an unlucky use of phase door.
Reproach is not half bad! It scales identically to willful strike, I leave it at 1 for a long, long time typically. Note it'll eventually trigger madness checks.
Hateful whisper: yes this can echo around back onto people it's worn off of, as long as there are things for it to bounce to and fro from. This makes it a supremely efficient hate to damage power in crowds, as long as you've got 3 points or so in it.
Agony: I probably do more damage with this ability than any other on doomed, and I don't usually lead with it. Repeatedly triggers madness checks. DON'T cast again on targets already afflicted with agony, ESPECIALLY on turn 5 of agony where it does it's highest damage amount- that's wasteful.
Madness: oh, it's so lovely at 1 point, and it's lovely with more. ALL mind damage checks madness- gestures, agony, that random + mind damage on hit from your gloves or something. Did I mention it gets checked PER? It's kind of like oprah, but you're giving away status effects.
Umm... they need a redesign. They're potent enough anyways, but heighten fear is actually misdescribed compared to implementation- it works if enemies see YOU for the turns listed. AKA creeping dark and heighten fear don't play nice. FANTASTIC shutdown on bosses, and panic is a lovely, lovely panic button or pace controller for fights. I think of it as a better banishment, but it's got a chance ot screw you if you use it too late.
I advise only playing with this tree if you're experimenting, not a serious run.
I tend to go 1/4/1/1 until mid thirties, and then pump to 3/4/1/1. DON'T kill harmless things asap, just keep feeding off them. 1 point in feed will keep you busy for a long time.
Combines nicely with darkness and the +movespeed. Note even with 1 point, you'll eventually be swinging madness check, stun check, cross tier effects check, for about half a turn spent on a gesture swing. I usually don't do this.
Unnatural Body talents:
1/5/1/1 is my usual spread, and only because the fourth tier power has that itty bitty chance to throw off poisons or diseases. Thereby meaning if I'm not swinging cure magical, those crazy lethal 30 turn mid-high damage diseases can be ignored and I can focus incription prep on other things.
Shrouds is lovely. Nightmares is lovely. Itemization sucks (what do you mean my level 6 edens guile find has curse of MISFORTUNE?). That cursed sentry skill gives you something to do with all that weaponry, is actually fairly damaging, and can make a hallways plug in a cinch. Playstyle choice, but I think throwing 5/4/1/1 is worht the investment nigh everytime.
Probably yes please. Doomed do it well- deflection shield loves this damage blocker stuff.
Okay I think that's it for my not so brief overview and general thoughts.
Playstyle builds: magestyle caster ( forces + punishments), melee hit and run (gestures + darkness), attrition summoner (punishments + shadows). I'm sure there's more- interestin genough most builds have significant overlap on the other builds. A doomed can switch styles pretty handily.