I'm not sure what parts of this build is concept vs effectiveness. I think you've spent a lot of time and effort going for Ranged Def with little thought to much else.
Having played Dark/Dark with, at one point, Super Speed, TP, and Flight, I found that I used TP to travel outside mishes and whenever I was immobbed or fleeing a team-wipe, Hover made TP travel a bit safer and gave me some help in combat but it was ultimately awkward in switching between TT's wide/short cone and NF's narrow/long cone. I never used Flight except for fun. Hover was for combat, and TP was so much faster for outside travel and SS was so much faster for inside travel that Flight simply didn't have a place.
I use SS+CJ now, which makes the TT/NF switch easy.
I initially went with Mu Mastery for concept reasons, but I think you made a mistake in skipping Soul Storm. Not only is it one of the most pretty powers in the game (and it gets a lot of 'what power is that?' comments from heroes in co-op zones), it doubles well with Pet Gaze to lockdown EBs and even AVs. A Devastation Hold Proc in Dark Blast can be fired between those two Holds and get all but the immune.
Personally, my experience with Dark Embrace was that it provided some desperately needed protection at a hell of an Endurance cost. With your slotting, it's essentially unuseable and a waste of a power slot.
You have a LOT of underslotted / set mule powers: Dark Servant, Shadow Fall, Dark Embrace and Darkest Night could all use help if you actually plan on using any of them. Health is also underslotted, Stamina could use more if you plan on using any of your toggles.
I really, really loved the look of the Fortuna Mistress. But she was often slow to respond, and then prone to drifting too far towards the next mob. In comparison, Fluffy is a battlefield genius, attacking where needed and almost-never aggroing a mob you didn't drag or drop him into.
So although I was sad to see her go, the switch to Mace Mastery yielded stronger results. The Disruptor didn't look as cool as the Fortuna, but it had better brains. It attacked more readily and survived longer.
Scorpion Shield puts building for S/L Def on the table, potentially even the softcap (although a Dark/Dark doesn't really need to aim for softcaps).
Focused Accuracy means you don't miss. You probably won't need it unless you're fighting Arachnos or an AV, but it's nice to have it for those occasions.
Web Cocoon is a second hold, although it's not as quick or anywhere near as pretty as Soul Storm.
If you can go with Mace Mastery and it won't hurt your concept, I'd say go for it and build for the S/L softcap.
If you remain with Soul Mastery and you like hover-blasting stick with the Ranged/AoE Def idea, but bring up the AoE. Ideally, you'd want to try for 35% Ranged and 20-25% AoE Def (with your Acc debuffs effectively bringing you to ~45% Ranged and 35% AoE Def). Putting triples of BotZ in your travel powers (Hover, Fly, Recall Friend, etc.) will help you considerably with that goal.
I'd strongly urge you to get Hasten and to give Fearsome Stare an Accurate To Hit Debuff set (pick the set with the Def bonus you'd prefer). FS slotted like this can be as potent an Accuracy debuff as Darkest Night but it can be slapped on every mob at a much cheaper End price (in addition to Fearing them).
I'd slot Tar Patch with more Recharge Reduction too, so you can hopefully use it with every mob.
Personally, I find that Dark Servant slotting almost a crime. Fluffy is too good to set mule him!
I'd seriously look again at Shadow Fall. It easily yield 5.6% Def to everything, which is better than any set bonus. And it extends that Def to Fluffy and any other pets or team-mates around you.
A stealth proc in Hover or Flight (or TP, Sprint, etc.) can double up with SF to make you invis. It's so easy to have that up for a Dark/Dark that it seems a shame not to.
This is a corruptor build I'm tinkering with - currently it's strong point is a 35% ranged def, and a lot of debuffs.
Villain Plan by Mids' Villain Designer 1.703
http://www.cohplanner.com/
Click this DataLink to open the build!
Nyx: Level 50 Magic Corruptor
Primary Power Set: Dark Blast
Secondary Power Set: Dark Miasma
Power Pool: Flight
Power Pool: Fitness
Power Pool: Teleportation
Ancillary Pool: Soul Mastery
Villain Profile:
Level 1: Dark Blast -- Thundr-Acc/Dmg(A), Thundr-Dmg/EndRdx(3), Thundr-Dmg/Rchg(5), Thundr-Acc/Dmg/Rchg(7), Thundr-Acc/Dmg/EndRdx(42), Thundr-Dmg/EndRdx/Rchg(43)
Level 1: Twilight Grasp -- Nictus-%Dam(A), Nictus-Heal(17), Nictus-Acc/Heal(17), Nictus-Acc/EndRdx/Heal/HP/Regen(25)
Level 2: Gloom -- Thundr-Acc/Dmg(A), Thundr-Dmg/EndRdx(3), Thundr-Dmg/Rchg(5), Thundr-Acc/Dmg/Rchg(7), Thundr-Acc/Dmg/EndRdx(43), Thundr-Dmg/EndRdx/Rchg(43)
Level 4: Tar Patch -- P'ngTtl--Rchg%(A), P'ngTtl-Rng/Slow(19), P'ngTtl-EndRdx/Rchg/Slow(19)
Level 6: Moonbeam -- ExecCtrt-Acc/Dmg(A), ExecCtrt-Dmg/EndRdx(11), ExecCtrt-Dmg/ActRdx(15), ExecCtrt-Dmg/Rng(25), ExecCtrt-Dmg/Rchg(31), ExecCtrt-Stun%(50)
Level 8: Tenebrous Tentacles -- TotHntr-Acc/Rchg(A), TotHntr-EndRdx/Immob(9), TotHntr-Acc/EndRdx(9), TotHntr-Immob/Acc(11), TotHntr-Dam%(21), TotHntr-Acc/Immob/Rchg(45)
Level 10: Hover -- Zephyr-Travel(A), Zephyr-Travel/EndRdx(34)
Level 12: Night Fall -- Posi-Acc/Dmg(A), Posi-Dmg/EndRdx(13), Posi-Dmg/Rchg(13), Posi-Dmg/Rng(15), Posi-Acc/Dmg/EndRdx(40), Posi-Dam%(45)
Level 14: Fly -- Frbd-Stlth(A)
Level 16: Swift -- Flight-I(A)
Level 18: Health -- Numna-Regen/Rcvry+(A)
Level 20: Stamina -- P'Shift-End%(A), P'Shift-EndMod(21)
Level 22: Fearsome Stare -- Abys-Dam%(A), Abys-Acc/Rchg(23), Abys-EndRdx/Fear(23), Abys-Acc/EndRdx(27), Abys-Acc/Fear/Rchg(27)
Level 24: Recall Friend -- RechRdx-I(A)
Level 26: Shadow Fall -- S'fstPrt-ResDam/Def+(A), S'fstPrt-ResKB(34)
Level 28: Petrifying Gaze -- Lock-Acc/Hold(A), Lock-Acc/Rchg(29), Lock-Rchg/Hold(29), Lock-EndRdx/Rchg/Hold(31), Lock-Acc/EndRdx/Rchg/Hold(31), Lock-%Hold(37)
Level 30: Howling Twilight -- Stpfy-KB%(A), Stpfy-Acc/Rchg(34), Stpfy-EndRdx/Stun(42), Stpfy-Acc/EndRdx(42), Stpfy-Stun/Rng(45), Stpfy-Acc/Stun/Rchg(46)
Level 32: Torrent -- FrcFbk-Rechg%(A), FrcFbk-Dmg/EndRdx/KB(33), FrcFbk-Rchg/EndRdx(33), FrcFbk-Rchg/KB(33), FrcFbk-Acc/KB(46), FrcFbk-Dmg/KB(46)
Level 35: Blackstar -- Cloud-ToHitDeb(A), Cloud-Acc/ToHitDeb(36), Cloud-Acc/Rchg(36), Cloud-ToHitDeb/EndRdx/Rchg(36), Cloud-Acc/EndRdx/Rchg(37), Cloud-%Dam(37)
Level 38: Dark Servant -- Lock-Acc/Hold(A), Lock-Acc/Rchg(39), Lock-Rchg/Hold(39), Lock-EndRdx/Rchg/Hold(39), Lock-Acc/EndRdx/Rchg/Hold(40), Lock-%Hold(40)
Level 41: Dark Embrace -- ResDam-I(A)
Level 44: Power Boost -- RechRdx-I(A)
Level 47: Summon Mistress -- ExRmnt-Acc/Rchg(A), ExRmnt-Acc/Dmg(48), ExRmnt-Dmg/EndRdx(48), ExRmnt-Acc/Dmg/Rchg(48), ExRmnt-EndRdx/Dmg/Rchg(50), ExRmnt-+Res(Pets)(50)
Level 49: Darkest Night -- EndRdx-I(A)
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Level 1: Brawl -- Empty(A)
Level 1: Sprint -- Empty(A)
Level 2: Rest -- Heal-I(A)
Level 1: Scourge
Level 6: Ninja Run