I use 2 Rechg 3 Tht_Debuf and 1 Immob (and run Hasten and Tactics) on my Thugs/Dark (PvE build). That way he's basically perma, doesn't miss much, has a nice ThT_Debuff which stacks with mine, and the immob is useful when fighting Heroes. 
CoX 50s: <ill/rad> <ice/ice> <fire/kin> <grav/sonic> <ice/storm> <earth/kin> <kin/elec> <cold/psy> <thugs/dark> <fire/dark> <dark/elec> <night widow> <EM/ninj> <mind/icy>
As you're thugs/dark, you should seriously kick butt at PvP. Stay in bodyguard mode as much as possible..shadowfall running to provide extra defence...fearsome stare when you're attacked and heroes and villains alike will crumble before your power.
People do not realise how dangerous a mastermind can be in PvP..pretty much the only way you can be taken down is to take out all your pets at once, leaving you vulnerable. Blasters/Corruptors using their Nova powers on a suicide mission against you is probably the thing that you most need to guard against, in my experience.
I love PvP, me!!
He has fairly high base accuracy and his -ToHit aura is autohit, but you'll probably still want to slot at least one Acc SO in the power since his heal and hold are both useful and require a ToHit check.
His tohit debuff is definately worth slotting since he has defender level debuff numbers. I wouldn't bother with slotting for immobilise though unless you have major trouble with "don't let X escape" missions in PvE.
For PvP you'll want to slot him for accuracy and to have him out as much as possible... he's just short of perma with 3 Green Recharge SOs. Personally I went with 1 Acc/3Recharge/2ToHitDeBuff on my Bot/Dark.
Ideal Slotting? 3 Membrane HOs (Tohit/Defense/Recharge) + 3 Endoplasm HOs (Acc/Mez). I wouldn't slot him for +Heal unless you really need more green numbers. His base heal is nearly as strong as your slotted heal anyway.
Dark Servant currently can't accept Pet IOs, so you'll probably be stuck with using the "common" level-50 IOs in I9 if you want to replace SOs. It'd be a decent increase on SOs though, +40% compared to +33.3% for schedule A enhancements like Accuracy. 
I have 1 acc, 3 recharge, 2 hold on mine and he's perma with hasten running. I solo'd about 97.5% of the time with my mastermind and its what worked for me. The hold probably gives me more as a Necro than you'd get from it and I'd probably put 2 -tohit instead
"Well, they found my diary today.
They were appropriately appalled
at the discovery of the eight victims
They're now putting it all together.
Women wrapped in silk
with one leg missing
Eight legs, one body, silk,
spider, brilliant!"
Thanks for all the input, once I have rached lvl 40 and gotten two more slots for him, I'll enter the PvP scene. (/em evillaugh)
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pretty much the only way you can be taken down is to take out all your pets at once, leaving you vulnerable. Blasters/Corruptors using their Nova powers on a suicide mission against you is probably the thing that you most need to guard against, in my experience.
I love PvP, me!!
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I think tp foe is the more common way and confuse on the MM should prove extremely funny as all his bodyguards turn on him.
Mind of Gaia lvl 50 Defiant's first Mind/Storm 'troller.
Deadly Doc 50 Dark/Dark Corr
and lots more on Pinnacle,Union and Defiant
Soul Drain and a critical hit on the MM works too btw
"Well, they found my diary today.
They were appropriately appalled
at the discovery of the eight victims
They're now putting it all together.
Women wrapped in silk
with one leg missing
Eight legs, one body, silk,
spider, brilliant!"
[ QUOTE ]
I have 1 acc, 3 recharge, 2 hold on mine and he's perma with hasten running. I solo'd about 97.5% of the time with my mastermind and its what worked for me. The hold probably gives me more as a Necro than you'd get from it and I'd probably put 2 -tohit instead
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this is pretty much how you should slot it even without hasten he is only not around for 5 seconds
Okay, Dark Servant. Nice.
Now, how to slot the damned thing, lol?
So far I have 2 recharge and 2 accuracy SO's and it's working peachy so far.
I was thinking of going 1 hold and 1 immob for the last two slots.
Worth noting is; I solo 97% of the time I play and was considering trying my hands at PvP with this MM.
I don't think the Servant needs any heals, since I have Twilight Grasp slotted with 3 heal SO's and 3 accuracy SO's
My primary is Thugs (big suprise, huh?)