When my opponent casts Mortify and in response I'm gonna cast Justiciar's Portal . Is that creature now illegal target for him? Or not? I'm not sure because it will be back again in the battlefield before Mortify will be top card on the stack.
Spells and abilities check for legality of their targets twice; once when you put them on the stack, and again when they resolve. If some, but not all, of its targets become illegal between the first and second check, the spell or ability resolves normally but does not affect or obtain information about illegal targets when it resolves.
When my opponent casts Mortify and in response I'm gonna cast Justiciar's Portal . Is that creature now illegal target for him? Or not? I'm not sure because it will be back again in the battlefield before Mortify will be top card on the stack.
A permanent that leaves the battlefield and then returns becomes a new object with no memory of its past exsitence on the battlefield, and anything on the stack that was targeting it no longer recognizes it as its target. Mortify will not resolve and it won't have any effect, because its only target is now illegal.
EDIT:
I found this text on internet:
Spells and abilities check for legality of their targets twice; once when you put them on the stack, and again when they resolve. If some, but not all, of its targets become illegal between the first and second check, the spell or ability resolves normally but does not affect or obtain information about illegal targets when it resolves.
Is this interpretation of rules correct?
That isn't the correct quote, that's for spells and abilities that have more than one target, and for which only some of the targets become illegal, but not all. Here is the relevant rule in full quoted from the Comprehensive Rules, emphasis mine:
608.2b. If the spell or ability specifies targets, it checks whether the targets are still legal. A target that's no longer in the zone it was in when it was targeted is illegal. Other changes to the game state may cause a target to no longer be legal; for example, its characteristics may have changed or an effect may have changed the text of the spell. If the source of an ability has left the zone it was in, its last known information is used during this process. If all its targets, for every instance of the word "target," are now illegal, the spell or ability doesn't resolve. It's removed from the stack and, if it's a spell, put into its owner's graveyard. Otherwise, the spell or ability will resolve normally. Illegal targets, if any, won't be affected by parts of a resolving spell's effect for which they're illegal. Other parts of the effect for which those targets are not illegal may still affect them. If the spell or ability creates any continuous effects that affect game rules (see rule 613.10), those effects don't apply to illegal targets. If part of the effect requires information about an illegal target, it fails to determine any such information. Any part of the effect that requires that information won't happen.
I'm a former judge (lapsed), who keeps up to date on rules and policy. Keep in mind that judges' answers aren't necessarily more valid than those of people who aren't judges; what matters is we can quote the rules to back up our answers. When in doubt, ask for such quotes.
Justiciar's Portal
I found this text on internet:
Spells and abilities check for legality of their targets twice; once when you put them on the stack, and again when they resolve. If some, but not all, of its targets become illegal between the first and second check, the spell or ability resolves normally but does not affect or obtain information about illegal targets when it resolves.
Is this interpretation of rules correct?
EDIT: That isn't the correct quote, that's for spells and abilities that have more than one target, and for which only some of the targets become illegal, but not all. Here is the relevant rule in full quoted from the Comprehensive Rules, emphasis mine: