I find this to be perhaps the less useful of all the traps cards. Sure being able to attack the enemy with any character no matter where it is, even in the staging area, can be quite the great boon, but it does nothing to help you againts that enemy until the player's attacks part of the combat phase. I almost always would prefer playing a Ranger Spikes, Entangling Nets, Ambush or Outmatched. That said in a dedicated Damrod trap deck it's a nice 0-cost drop if you have nothing else to play, so you could go ahead and include it.
Play Ithilien Pit into the staging area unattached.
If unattached, attach Ithilien Pit to the next eligible enemy that enters the staging area.
Any character may choose attached enemy as the target of an attack.
Encounter at Amon Dîn #64. Lore.
It's great for a secrecy deck. Think about it - normally enemies may pile up in the staging area if you are in secrecy. With this trap, you can take care of them in the combat phase, without having to engage them.