Noble Phantasm: Trojan Horse (N/A)
A last ditch attempt at victory, this Noble Phantasm calls on Odysseus' final act in the Trojan War that led to the true beginnings of his legend. After battling a single Servant and Master ten times without a clear victor on either side, the last time he escapes, he leaves behind an item for the opponents, a gift of sorts that the Master or Servant are compelled to accept and take it home. From here on out, target's own luck is equal to his own.
They will be given three warnings from their closest ally, magus or not, about the nature of the gift, and each time they will roll their luck stat against Odysseus' own. With each loss, their luck is reduced by a rank while they grow more incredulous of their ally, with each victory, it is Odysseus' own luck that reduces by a rank, and the target begins to gain more clues behind the gift.
That night, if the target's luck is lower than Odysseus' own, at the target's most inconvenient moment, the true nature of the gift is revealed as Odysseus and a number of heroic spirits summoned for just this one attack explode from it, all defenses within the target's base are completely shattered and anyone can enter. All stats of both sides equal their own current Luck stat. However, if it is the target with the higher luck, then only Odysseus appears for the final surprise attack and the battle proceeds as normal.
If the target knows his True Identity, then the Noble Phantasm cannot be used in the first place.
The keyword here is: Deception.