It affects lieutenants so Mag 3.
Dunno about the Magnitude but it affects Lieutenants for sure.
Copied this from nofuture.org.uk, has some really good stuff on so hope this helps 
Level 50 Defender Effects
* 22.35 second Terrorized (magnitude 3) to Target if (target = critter)
* -18.75% ToHit to Target for 20 seconds
* 14.9 second Terrorized (magnitude 1) to Target (20% chance) if (target = critter)
* 10.43 second Terrorized (magnitude 3) to Target if (target = player)
http://coh.nofuture.org.uk/powersets/powersets.php
Mag 3, with a chance to crit. It'll hold lieutenants always when it lands, and bosses very occasionally.
DON'T eat muffins while I'm developing you.
Pants! Turkey Magnates! A man in a box! Rogue AI! Come one, come all, to arc ID 10107 - It's [i]'Not as long as some other arcs'[/i]!
Thanks all for your answers.
Black Mage: yes, I know Nofuture. It is one of the sources of the discrepancy I mentioned. Nofuture is "Red Tomax's guide". And it is the one with the correct data, btw.
Wilfred: I thought that "critical controls" in which the magnitude is incremented were only possible for controllers. Good to know that defenders can have them too. In fact, looking again at Nofuture's data, seems obvious that defenders can have them too...
[ QUOTE ]
Thanks all for your answers.
Black Mage: yes, I know Nofuture. It is one of the sources of the discrepancy I mentioned. Nofuture is "Red Tomax's guide". And it is the one with the correct data, btw.
Wilfred: I thought that "critical controls" in which the magnitude is incremented were only possible for controllers. Good to know that defenders can have them too. In fact, looking again at Nofuture's data, seems obvious that defenders can have them too...
[/ QUOTE ]
Its Power Based rather than AT based.
The Crit means you can occasionally fear Vash Zombies too as well.
EMP is another power which can Crit on a defender, I dont know if there are any more than those two though
"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!"
Cyn, I am starting to think you know more things about the game than MaX 
I have found a discrepancy between the data in Paragonwiki and Red Tomax's.
The magnitude of the Fearsome Stare power (from Dark Miasma) is 2 or 3? In short, does it effect lieutenants?
(Paragonwiki says it is 2 and Red Tomax in Nofuture says it is 3)
As I am not at home, can't test that so if you can answer quickly I would be very grateful. I am planning a build and, though I have the power, I can't remember...
Thanks for your help!