Apologies if this is answered in some buried thread...
Black Riders, A Shadow of the Past, Stage 2, A Shortcut to Mushrooms
" When a player fails a Hide test, each Nazgul enemy in the staging area engages that player."
Okay.
You're staging. Card from encounter deck is revealed, a Nazgul (Black Rider)
" Hide 2. Cannot have non- Morgul attachments.
Forced: After engaged player fails a Hide test, Black Rider makes an immediate attack."
You do the Hide test. You fail it.
Does this Black Rider you have in your hand engage you (per Stage 2A) or not?
I believe it does not: it is not in the staging area, as it is being revealed still.
But some folks on other threads seem to think it will engage you.