Nightmares Upon Nightmares

Name: Nightmares upon Nightmares
[Gehenna:R]
Cardtype: Event
Gehenna. Do not replace until your next discard phase.
For each minion a Methuselah controls during his or her unlock phase, the Methuselah must burn a card from his or her hand or lock that minion. Cards burned in this way are not replaced until the master phase. Vampires with capacity greater than the number of Gehenna cards in play and mortal allies are not affected by this card.
Artist: Mark Nelson

When NuN is in play, how does the burn card/lock effect interacts with cards that come to play on untap phase like Emerald Legionnaire or a previously contested minion who returns to play after other contestors yield?

When NuN is in play, how does the burn card/lock effect interacts with cards that come to play on untap phase like Emerald Legionnaire or a previously contested minion who returns to play after other contestors yield?

INA, but I'd say you cannot end your unlock phase or pass the impulse if you control a minion you haven't locked or burned a card for. So if you get a minion back from contest or a Legionnaire back from the Ash Heap, you must either lock them or burn a card.

See this ruling on Arika's ability:

Arika's prey cannot end her unlock phase if she controls a location that she hasn't burned a pool for, even if she gained control of that location during that unlock phase. [LSJ 19990405]

Yes, NuN is an on going effect. You don't "resolve NuN" and then do something else.

It is rather like each minion has an effect attached as long as NuN is in play that say you have to either lock it or discard a card for it.

How does NuN interaact with infernal minions?

my understaning so far was, that i can decide to "lock" them, while they are already locked (since the do not automatically unlock at the beginning of the unlock phase) and then can unlock them voluntarily with the infernal payed unllock.

This works in my mind, since the "regular" unlock must be the first thing in the unlock phase by the rules, and after that in can order any effects like I please as the active player.

NuN for hand management in a nocturn deck is nice.