It certainly looked like the passive of Osiris's [God Card].
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[Nether God: Wields death, proclaims eternal life. Osiris's Black Spirit Vulture is an immortal Nether God. Even if it falls into the underworld, it is merely the beginning of a new cycle of rebirth. Magic Power +2000, Attack +2000, Defense +2000, Rank +2. Immune to most transformative Ultimate Moves, and completely immune to instant-death Ultimate Moves. Additional Ultimate Move: Immortality.]
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"Immortality..."
The boosts to the three core stats granted by [Nether God] were comparable to those of [Thunder God], and should be the standard bonuses of a [God Card].
Its complete immunity to instant-death Ultimate Moves, meanwhile, was likely a standard trait of [Nether God].
As for [Immortality], it was most likely Osiris's Black Spirit Vulture's unique ability.
Osiris's Black Spirit Vulture wielded authority over death and rebirth. Even after being transformed into a Magic Spirit, it could use that authority upon itself, allowing it to cycle endlessly between death and rebirth. A thirty-second revival delay could hardly be considered a drawback.
At the same time, this meant that after dying, it would remain in the [Graveyard] for thirty seconds, allowing it to interact with effects related to the [Graveyard].
Moreover, Osiris's Black Spirit Vulture's [Immortality] possessed an unconventional trait: even outside of battle, if it was offered up as a sacrifice, it could still revive thirty seconds later.
This meant that even if it were used merely as a sacrifice for a twelve-star Magic Spirit, it had tremendous potential!
Outside a duel, Dark could use it every thirty seconds to Tribute Summon a [Demon King].
That would greatly simplify the steps needed to Fusion Summon [Demon God: Abyss Walker].
This one Ultimate Move alone was enough to amaze him. Even if all the remaining Ultimate Moves were