Tar Patch helps you (and all team mates) kill enemies faster by increasing the damage they'll take from any attacks while in the tar patch.
Darkest Night is one of the best debuffs in the game, and some would say THE best, and what it mainly does above all else is stop them hitting you by MAJORLY reducing their ToHit chances.
Darkest Night burns a lot of endurance until you can slot it, but it is extremely useful in PvP as well as in PvE. Darkest Night is a toggle, and lasts as long as the target of it, which is known as the anchor, is still alive. Tar Patch isn't really much cop in PvP, but as a PvE attack, it does help hold mobs in place, prevent escape, as well as making them all take more damage. Tar Patch helps keep the rest of a mob in the area of the anchor of your Darkest Night.
You definitely do want both, the only question is which to take first. Your choice really.
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Alot of lower levelled new players don't appreciate the power that these debuffs have... I find it at outmost annoyance that they don't recognise them even if I just tell them how it works... Usually no matter what I do they would kill my anchor first, or definately not last, putting the team at a severe disadvantage. 
I must recommend Darkest Night. Not only does it more or less floor your opponents' to hit chances but it also debuffs their damage to an extent that even a tanker would envy. It costs a fair bit of endurance for sure but with two DOs, it's not that bad. I only run into endurance problems when I run both DN and Shadow Fall at the same time (defender hero). In my opinion, Darkest Night is the reason to pick Dark Miasma in the first place.
Like others have posted, I would try and work both into your build. Both are very good debuffs.
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Thanks guys. I definatly know now wich two powers I will take next. I'm just unsure in which order. 
Another vote for both. In fact, I would say you should've had both by level 6. They're the two best powers a dark/dark can get. As to which first, DN, no question. Best PvE debuff in the game.
Problem is when I solo I curently have enough endurance problems (in longer fights). So I might go with Tar Patch first. Then I will Darkest Night. And with 20 Endurance.
I took Tar Patch first, as I had no idea how Darkest Night would work. I had the impression I wouldn't be able to do anything else but channel Darkest Night (sorry, I played WoW, and channeling ment you could do nothing BUT channel).
On the endurance cost side: Darkest Night is cheaper than sprint. So, if you have endurance problems, turn sprint off. It should help... a lot!
Aye, DN ain't a particularly end-heavy toggle, plus you'll be saving end by not having to use TG so often
I agree with everyone else here. Both are excellent powers. Take them, and most importantly, give them some hot slot loving. Darkest night with three to hit debuff and one or two endred SOs is awsome. I'd also recomend puttting three recharges in tar patch, as you can stack them for some awsome res debuffing.
Dead Calm's Defender Manifesto
Both, but Tar Patch as priority. Keeps things at a distance and seriously reduces their resistance, what more could a corruptor want?
"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!"
/agree. Spot on. DN slotted 3x End Red and 3x Acc Debuff ftw!
Def take tar patch, i've gotten out of some really tight spots with that lil beauty, plus in a team, it really helps with pulls. (pull but have TP ready round a corner)
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/agree. Spot on. DN slotted 3x End Red and 3x Acc Debuff ftw!
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I wouldn't slot 3 end red in DN (unless with TO's). 1 end red SO and 3 slotted stamina is all you need (and 3 tohit debuff SO's ofcourse).
Slap DN on the lt or boss, forget about him and kill the minions, then take on the debuffed target.
Tar patch is something I would take asap. Especially if you are all about AoE. Some AoE powers that are otherwise fairly useless (like Rain of Fire) are becoming real killers with tar patch. Put 3 recharge SO's in it.
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Hi, just a questin. I have a dark/dark corruptor whos level 14 at the moment. But I still don't have taken Tar Patch or Darkest night (Nontheless I'm doing fine and am capable of soloing my missions). There were always some other Power I prefered first. On level 16 I will take one of those. Which is better to take first? I play PvE mainly.
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Both deffo! for teh most part the whole DM set id fab tho the juries out on black hole its hard in teams trying to explain over and over that some baddies are in phaze *yawns*
Personally for soloing it might depend on ur setting, if ur goin for large minion mobs then tar deffo its really handy and it used to fek up paragon protectors in MOG. I love the lil red shields =)
but to me i'd think darkest night, wack it on the boss or furthest away minion and blast away. Mind to kill said anchor last tho 
Hi, just a questin. I have a dark/dark corruptor whos level 14 at the moment. But I still don't have taken Tar Patch or Darkest night (Nontheless I'm doing fine and am capable of soloing my missions). There were always some other Power I prefered first. On level 16 I will take one of those. Which is better to take first? I play PvE mainly.