If you use the ability of Ithilien Tracker..., and when encounter cards are revealed:

1. First card is treachery that kills Ithlien Tracker

2. Second card is an enemy

That enemy's threat is 0 by Ithilien Tracker's effect? Or not because he isnt in play? The effect of Ithlien Tracker remains?

In that scenario, you should still be to lower the next enemy's threat. You've already paid the cost and activated the ability, and nothing appears to hinge on the Tracker remaining in play.

Ok, thanks for the clarification.

Mm, what happens if

1. its an enemy which is immune to player card effects

2. its en enemy

What does the tracker do?

It shouldn't have any effect.

First enemy is the only one who should be eligible, and he cannot.

Mm, what happens if

1. its an enemy which is immune to player card effects

2. its en enemy

What does the tracker do?

Wow, this is a good question. And probably no effect then, which is tough on the tracker. However, most of the immune enemies do not enter during staging, right?

Mostly they don't, but Morgul Wraith from Land of Shadow lives in the encounter deck and is immune when the Ring is exhausted. There are also some immune enemies that don't start in the encounter deck but can end up in the discard pile, such as the Witch-King from Black Riders. Finally, some enemies can enter the staging area through card effect during questing (Naurlug) or by advancing the quest mid-way through staging (rare, but can be done).