Something like this could work.
Hero Plan by Mids' Hero Designer 1.401
http://www.cohplanner.com/
[u]Click this DataLink to open the build![u]
Level 43 Magic Defender
Primary Power Set: Dark Miasma
Secondary Power Set: Dark Blast
Power Pool: Flight
Power Pool: Fitness
Power Pool: Leadership
Hero Profile:
Level 1: Twilight Grasp -- Acc(A), Acc(3), Heal(3), Heal(5), Heal(5), RechRdx(7)
Level 1: Dark Blast -- Acc(A), Dmg(7), Dmg(9), Dmg(9), EndRdx(11), RechRdx(11)
Level 2: Gloom -- Acc(A), Dmg(13), Dmg(13), Dmg(15), EndRdx(15), RechRdx(17)
Level 4: Tar Patch -- RechRdx(A), RechRdx(17), RechRdx(19)
Level 6: Darkest Night -- ToHitDeb(A), ToHitDeb(19), ToHitDeb(21), EndRdx(21)
Level 8: Shadow Fall -- EndRdx(A), DefBuff(23), DefBuff(23), DefBuff(25), ResDam(25), ResDam(27)
Level 10: Hover -- EndRdx(A)
Level 12: Fearsome Stare -- Acc(A), ToHitDeb(27), ToHitDeb(29), ToHitDeb(29)
Level 14: Fly -- Flight(A)
Level 16: Swift -- Run(A)
Level 18: Health -- Heal(A)
Level 20: Stamina -- EndMod(A), EndMod(31), EndMod(31)
Level 22: Tenebrous Tentacles -- Acc(A), Dmg(31), Dmg(33), Dmg(33), EndRdx(33), RechRdx(34)
Level 24: Night Fall -- Acc(A), Dmg(34), Dmg(34), Dmg(36), EndRdx(36), RechRdx(36)
Level 26: Howling Twilight -- RechRdx(A)
Level 28: Maneuvers -- DefBuff(A), DefBuff(37), DefBuff(37), EndRdx(37)
Level 30: Tactics -- ToHit(A), ToHit(39), ToHit(39), EndRdx(39)
Level 32: Dark Servant -- Acc(A), ToHitDeb(42), ToHitDeb(42), ToHitDeb(42), Heal(43), Heal(43)
Level 35: Life Drain -- Acc(A), Heal(40), Heal(40), Heal(40)
Level 38: Vengeance -- DefBuff(A)
Level 41: [Empty]
Level 44: [Empty]
Level 47: [Empty]
Level 49: [Empty]
------------
Level 1: Brawl -- Empty(A)
Level 1: Sprint -- Empty(A)
Level 2: Rest -- Empty(A)
Level 1: Vigilance
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Yikes! Too many slots in Shadow Fall. I run with the Steadfast: KB Protection and one EndRedux. The defense isn't impressive enough to warrant six slots.
Everything else... sure, that could work. I do things a little differently than that, but to each his/her own. Dark is awesome, regardless of your build. Just read up on Jock Tamson's guide and understand what the powers do, and you're golden.
EDIT: Have fun. You're going to love this toon! 
[ QUOTE ]
Yikes! Too many slots in Shadow Fall. I run with the Steadfast: KB Protection and one EndRedux. The defense isn't impressive enough to warrant six slots.
[/ QUOTE ]
By slotting their stealth aura for defense and getting manuevers, storm summoners and dark miasmists get a little over 13% defense. This is defense that is being applied to the entire team (within range) to all defense types including psychic. This is especially noteworthy because they are both sets that do the majority of their damage mitigation through enemy disruption. Adding such a significant amount of defense on top of this makes them capable of covering the survivability needs for just about any team.
Fair enough, but is that really going to be necessary? With all of the stacking -tohit and the multiple effects from Tar patch, the extra defense isn't what's going to be saving lives out there. I guess it's all up to personal preference, but I've never taken the Leadership pool nor slotted Shadow Fall more than twice, and people don't die on my D3's teams. Just seems to be a large tradeoff for a mediocre benefit that you can effectively do without.
When in Rome... 
Well if no one dies either way does it really matter?
Good point.
There's definitely something to be said for layered protections. The benefits of -tohit are just that much better when you backstop it with a solid chunk of defense and some resists.
I won't take Leadership pool, a D3 already adds a lot to a team without having maneuver/tactics sucking up even more endurance. Do your debuffs, and let loose with the aoes... and any team should be happy enough with just that.
But if you want, a good slotted Shadow Fall and Maneuvers does help on team defense.
/Empaths can turn three people into Jesus, one person into God, and everyone else into the twelve apostles.~Angry_Citizen
Don't you know that discussion of power selection/slotting can ONLY be based on hearsay, rumor, idle speculation, and bald-faced lies??!? ~Elf_Sniper
I've read the guides but some of them seem outdated. The most current guide doesn't have a build- can someone post one for me? I'm new so its helpful to see what other folks use. This should be a 'levelling build' because since I'm not a twink, so I don't have any fancy snap ins, IOs etc