Nightmare, Yixlid Jailer, Corpse Lunge Interaction
0 damage.
Corpse Lunge moves an object with the sole purpose of using information about that object, so it looks at what the object is like in the zone it has just left (this is the opposite of the case where you move an object and then do something else to it, in which case you look at the object in the zone it goes to to get information about it). The last time Nightmare was in the graveyard, its power/toughness was 0/0 for the reasons you state. Hence Corpse Lunge will use 0 for its effect.
Daniel Kitachewsky
L3, Paris, France
Rules NetRep