Let's say you're an Assault Marine with the 'Terror By Thy Friend' Power Armor History (Fear 1 foes in melee).

You jump pack Charge into a Magnitude 20 Horde of Renegade Militia.

How much magnitude damage loss should they fail their Willpower Test?

In general, how would Fear interact against Hordes?

I would suggest losing... 5 (seems reasonable) Magnitude for the failed Willpower, with an additional 1 Magnitude loss per Degree of Failure.

Hordes get a bonus against Pinning tests equal to their Magnitude. You could extend that same logic to Fear tests, if you wanted.

Also, a squad of Space Marines can only lose Cohesion once per encounter with a given Fear-causing creature (pg 277). You could also apply that principle to Hordes and sources of Fear.

Along those lines, you could treat Magnitude damage the same as Cohesion damage from Fear. Which would result in the loss of Magnitude equal to the Fear rating of the Marine (in this case 1).

That's not a lot of Magnitude loss though. And despite the fact that you aren't rolling an attack to make it happen, it would sit better with me if it were more impressive.

On the other hand, the rules for Fear seem to be geared toward "things that scare you," as opposed to "you scaring things." Ultimately, I dunno. It's much more cut-and-dried in Solo Mode, from what I've read.