Hi 
Only real comment is on the epic dark powers...
Soul Drain and Dark Consumption are probably better slotted with 1 x acc and 3 x recharge (soul drain is prolly ok 1 x ACC and 2 x recharge)
If you can then find some slots to put on Dark Consumption for end mod even better (might mean taking them in a different order)
I designed a disorient focused D3 build a while back while faffing about with heroplanner, which iirc was fairly similar to yours, but I've never got around to testing it out so I'd be interested to find out how it goes.
I agree with Psygon on the slotting of your ancillery powers (though I'd go with 1xAcc, 3xrecharge in Soul Drain). Also, I don't think oppressive gloom needs end reduction, as it's end cost is tiny. The health cost shouldn't be a problem either, health should cancel it out. Having said that, I've only ever used OG on test, so could be wrong.
Dead Calm's Defender Manifesto
Certainly a different build from most D3's
One thing I did think of tho.. If you are focusing of the disorients and taking all 3, (OG, DP & HT) then disorientating minions/lts shouldn't cause any problems. A boss may or may not become disorientated. With this in mind is there going to be any need for fearsome stare? Now I love fearsome stare and would never recommend anyone take it out of a 'normal' build but in your situation I can't really see it being that important, things dont need to be feared and disorientated. Also when you come to the later part of the game facing AV's you cant disorient or fear them, but you can hold them. Also if a boss isnt disorientated, try and hold him with PG, with fluffy aswell it should be easy enough. With PG you'll also be able to hold AVs etc. You aren't slotting FS til late anyway so maybe skip it all together? Take howling twilight in its place so you will have both your disorient moves early on to work together, then take petrifying gaze at lvl 28 and you've got a back plan for bossess and AV's.
Just a thought I had anyway
Thanks for the comments all. Re FS - I tend to use it as an emergency power to keep accidentally aggroed mobs at bay for a while, but it's worth thinking about the idea of being able to hold bosses in the later game, you're right. I just found PG annoying for some reason - I guess, being used to the speed of some controller holds (e.g. Mind, which I'm also playing on and off), the animation/activation was frustratingly long for me - but from a practical point of view you might be right about respeccing it in. I might try it on test.
Fearsome stare is a great ToHit debuff though, I'd keep it just for that.
I've been playing my DDD a lot recently, and after a respec at 35 I've settled on the concept of focussing on AoE attacks and Disorientation as the main "themes" of the buld, apart from debuffing mob accuracy (which is of course the number one priority for Dark Miasma and the main secondary effect for Dark Blast).
The DM/DB powersets are so good that it's hard to know what to leave out. In the end, after finding I wasn't using them that much, I respecced out of the single target attacks (Gloom, Moonbeam), which gave me room to add the leadership pool, the bonus of which for Defenders is so good it's hard to pass up. I also decided that since Fluffy would be spamming Petrifying Gaze, I could afford to leave it out (I found I wasn't using it much anyway in the style I was playing). This gave me room to include Recall Friend, which apart from being very team-friendly (even in the later levels, I find), enables me to teleport my Fluffy friend around if need be.
Re the disorientation: after playing around a fair bit, I've found that Howling Twilight 3-slotted with SOs comes up often enough to use fairly regularly in combat, nearly once a battle. The combination of Howling Twilight and Dark Pit is pretty awesome, and I'm looking forward to using Oppressive Gloom to stack the magnitude even higher, hopefully to catch bosses.
My main luxury has been 3-slotting Hover and Fly. I find that 3-slotted Hover is actually so useful in combat that it's worth getting it as early as reasonably possible. As for 3-slotted Fly - well, it just feels great!
Anyway, just thought I'd put it up here for discussion - any comments, insights, etc., would be greatly appreciated!
Exported from version 1.5C of CoH Planner
http://joechott.com/coh
Archetype: Defender
Primary Powers - Ranged : Dark Miasma
Secondary Powers - Support : Dark Blast
01 : Dark Blast endred(01) acc(3) dam(7) dam(9) dam(36)
01 : Twilight Grasp endred(01) acc(3) acc(5) hel(33) hel(34)
02 : Tar Patch endred(02) recred(15) recred(15) recred(31)
04 : Darkest Night endred(04) thtdbf(5) thtdbf(9) thtdbf(11)
06 : Hover fltspd(06) fltspd(7) fltspd(17)
08 : Shadow Fall endred(08) damres(11) damres(34) damres(33)
10 : Dark Pit acc(10) acc(13) endred(19) disdur(43) disdur(46)
12 : Fearsome Stare acc(12) acc(13) endred(19) ferdur(43) ferdur(43) ferdur(46)
14 : Fly fltspd(14) fltspd(17) fltspd(40)
16 : Swift runspd(16)
18 : Health hel(18)
20 : Tenebrous Tentacles endred(20) acc(21) dam(21) dam(27) dam(36)
22 : Stamina endrec(22) endrec(23) endrec(23)
24 : Night Fall endred(24) acc(25) dam(25) dam(27) dam(36)
26 : Assault endred(26)
28 : Howling Twilight endred(28) recred(29) recred(29) recred(33) dam(40) disdur(42)
30 : Tactics endred(30) thtbuf(31) thtbuf(31) thtbuf(34)
32 : Dark Servant acc(32) thtdbf(37) thtdbf(37) thtdbf(37)
35 : Recall Friend recred(35)
38 : Blackstar endred(38) acc(39) dam(39) dam(39) dam(40)
41 : Oppressive Gloom endred(41) acc(42) disdur(42) disdur(46)
44 : Dark Embrace endred(44) damres(45) damres(45) damres(45)
47 : Soul Drain endred(47) acc(48) dam(48) thtbuf(48)
49 : Dark Consumption endred(49) acc(50) dam(50) endrec(50)
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01 : Brawl acc(01)
01 : Sprint runspd(01)
02 : Rest recred(02)