The major thing to note here is that they are not accuracy debuffs, but to hit debuffs. This has some implications for the maths regarding the to-hit chages in I7, but for gods sake don't ask me to explain why. 
There are some very good stats people on the US boards, so I'd have a look there for some background.
Darkest Night
Accuracy - People thought this was 35%ish for ages, which was only put to bed when Castle posted it was around 18.5%.
Damage - Bang on
Twilight Grasp
Don't know if there are any stats anywhere for TG, so seriously appreciate these. It also debuffs the targets health regen.
Shadowfall
Last I heard the numbers are 22%res and 3.5%def, but I could be well out.
As for testing FS, I'd suggest council mech men, as they resist the fear aspect, but afaik the debuff should still work. Nemesis troops will also be an option in a few levels time.
I'd be really interested in the FS numbers, so if you need a hand testing, give me a shout.
Dead Calm's Defender Manifesto
[ QUOTE ]
The major thing to note here is that they are not accuracy debuffs, but to hit debuffs. This has some implications for the maths regarding the to-hit chages in I7, but for gods sake don't ask me to explain why.
[/ QUOTE ]
It's all a bit beyond me too, hence possible terminology problems. 
The hero planner I looked at gave 25% res for SF but as it was wrong about DN I wanted to test it. That was only one simple test of being shot by charged bolts with SF on and then a shot with it off. No reason the figure should be wrong but I'll do another couple of test against a different mob type to confirm it.
I wasn't planning on listing all the power effects (such as the minus hp regen of TG or the % heal) but I guess it could be nice for any new people to have all the facts in one place so I might just double check some of that stuff too. Any additions by other people welcome.
As it was sort of what kicked me off testing, this spreadsheet linked to here is quite handy.
I'll find me some mech men later.
I'm coming up with 22% from memory, so yours is by far more reliable.
Dead Calm's Defender Manifesto
More tests done and some interesting results (with correct terminology). 
Twilight Grasp
Damage Debuff - 12.5%
To Hit Debuff - 5-6%
Debuff Duration - 20 seconds
Heal - 23.5% of base hp
As TG is not auto-hit there is some inaccuracy for the to hit debuff. The figure I got was 16% (100 attacks as the sample size) and I'm fairly sure that is triple stacked so the 5% debuff seems most likely. Using hasten and running DN too, I couldn't seem to debuff the damage above 75%. I seem to remember defenders damage res cap is 75% so this could be the debuff cap too?
With TG triple stacked, an even level minion showed no hp regen at all.
Shadow Fall
Damage Resistance - 23.6%
Defence (ranged) - 0%
The sample size for the defense test was 164 ranged attacks. There were 83 hits. This really suggests there is zero ranged defence. A very brief test suggests the same for melee.
Fearsome Stare
I have FS down as 20 seconds duration and 40 seconds recharge. 3 slotted with recharge SOs should have made it near perma. Against an even level mech man, it showed absolutely no to hit debuff. I can only guess the debuff only works for targets that are affected by the fear effect. I might test this at some point.
Rounding up...
I did some more tests for fearsome stare and found that there is a to hit debuff for those that are affected by the fear (very dull tests of fear it, shoot it, hope it attacks back etc..). A small test against Council gave me a figure of about 7-11% (it was only a small test). Against Tuatha and Fir Bolg the figure was closer to 30%. Very odd...
As the damage debuff changes for level I've put the figures for DN and TG below.
Darkest Night
0 - 37.5%
+1 - 33.75%
+2 - 30%
+3 - 24.375%
+4 - 18%
+5 - 11.25
Twilight Grasp
0 - 12.5%
+1 - 11.25%
+2 - 10%
+3 - 8.125%
+4 - 6%
+5 - 3.75%
[ QUOTE ]
Darkest Night
Accuracy - People thought this was 35%ish for ages, which was only put to bed when Castle posted it was around 18.5%.
Damage - Bang on
[/ QUOTE ]
Really, I thought that Castle ( or someone ) stated this was 37.5%. Do you have a link?
Also if you put a tar patch down, the damage debuff gets multipled by the res debuff of tar patch ( 1.3 for corruptors so I'd expect 1.375 for defenders ).
@Unthing ... Mostly on Union.
Linky
I did wonder why Tar Patch said it was -dmg on the hero planner but wasn't when I tested. The multiplier is 1.3 it seems.
Just finishing up the figures for completion sake. Been testing Fluffy and his powers.
Fluffy Darkest Night - 80% as effective as your version for damage debuffing, ie. 30%. I'll assume the to hit debuff is also the same scale so 15% debuff.
Fluffy Twilight Grasp - Same 80% effectiveness for damage debuff so 10% damage debuff and 4% to hit debuff. Curiously it heals for 23.5% the same as the defender version and uses your hp not Fluffys (or so it seems).
Looking forward to going up against some AVs now as it should be fairly easy to debuff their damage down to 10% by the looks of it. Will be nice if I can hit an AV harder than it can hit me. 
I started a Dark/Rad a couple of weeks ago and I always get curious about the figures for the powers - just so that I don't end up slotting stuff that's not worth slotting etc.. I looked at the hero planner and at some guides on the US forums to check these. It was only today that I found the figure for Darkest Night was totally wrong so I decided to do some testing of my own. As I haven't seen too much on these forums about any of these numbers, I thought I'd post them here for everyone. Sorry if they are already around somewhere.
Darkest Night
Accuracy Debuff - 18.75%
Damage Debuff - 37.5%
Twilight Grasp
Damage Debuff - 12.5%
Accuracy Debuff - 7%
The accuracy debuff was a little harder to test. I confirmed that the damage debuff does stack and I could stack 3 on my target (with maybe a very tiny window where there were only two stacked). The small test I did for accuracy gave me a figure of 22%. Assuming accuracy also stacks, that works out nicely at about 7%. I'd have to do a bigger sample to get a more accurate figure.
Shadow Fall
I only had time to run one very quick test with regards to the damage resistance provided. That worked out at 23.5%. I'll do some more tests when I next have the chance and also test the defence bonus it gives.
I also want to test the accuracy debuff in Fearsome Stare at some point. Anyone know any good mob types that are resistant to fear (and a lvl26 can last more than 4 seconds against)?