Benjen Stark

Reasonable enough for the stealthy bicon, his ability can be useful to a handful of decks, but is largely passed over for more widely applicable cards in most constructed decks.

question,i shuffle him back in or the effect happens before he go to dead pile?

The effect happens first.

Slightly longer answer : Benjen becomes Moribund:Dead. Then the shuffle happens. Then Benjen goes to the dead pile.

VonWibble might have answered this allready, but I just want to make sure I got it right..
If I have Benjen Stark + some other characters in play and Valar Morghulis plot is revealed. Am I doing the shuffle first, and then put all characters that was in play, into the dead pile? Or is it just characters with some "when killed" ability that will enter the dead pile after the shuffle has happened?

Say you are attacking a stark player with Benjen and a few other characters in a MIL challenge. As their player action, the Stark player plays Lethal Counterattack (Core) to kill all your characters - Benjen included.

Step 1 Initiation: Event is played
Step 2 Save/Cancel: You have no cancel for the event nor any saves for the kill effect
Step 3 Resolution: All your attacking characters become moribund:dead pile but none leave play yet
Step 4 Passives: Any passives with conditions that have been met activate (Such as Priestess of the Pyre (ASoS) could stand if it is Summer)
Step 5 Responses: Players take turns triggering responses to any of the previous steps. Here is where you get to trigger Benjen and each player shuffles their current dead pile back into their decks.
Step 6 Clean-up: Moribund cards leave play, so all your attacking characters are now placed into the dead pile.