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If an ability would produce one or more mana of an undefined type, it produces no mana instead.

Some abilities produce mana based on the type of mana another permanent or permanents “could produce.” The type of mana a permanent could produce at any time includes any type of mana that an ability of that permanent would produce if the ability were to resolve at that time, taking into account any applicable replacement effects in any possible order. Ignore whether any costs of the ability could or could not be paid. If that permanent wouldn’t produce any mana under these conditions, or no type of mana can be defined this way, there’s no type of mana it could produce.

To “tap [a permanent] for mana” is to activate a mana ability of that permanent that includes the {T} symbol in its activation cost. See rule 605, “Mana Abilities.”

An ability that triggers whenever a permanent “is tapped for mana” or is tapped for mana of a specified type triggers whenever such a mana ability resolves and produces mana or the specified type of mana.

A replacement effect that applies if a permanent “is tapped for mana” or tapped for mana of a specific type and/or amount modifies the mana production event while such an ability is resolving and producing mana or the specified type and/or amount of mana.

If an object’s enters-the-battlefield triggered ability or replacement effect refers to X, and the spell that became that object as it resolved had a value of X chosen for any of its costs, the value of X for that ability is the same as the value of X for that spell, although the value of X for that permanent is 0. This is an exception to rule 107.3i.

The tap symbol is {T}. The tap symbol in an activation cost means “Tap this permanent.” A permanent that’s already tapped can’t be tapped again to pay the cost. A creature’s activated ability with the tap symbol in its activation cost can’t be activated unless the creature has been under its controller’s control continuously since their most recent turn began. See rule 302.6.

The untap symbol is {Q}. The untap symbol in an activation cost means “Untap this permanent.” A permanent that’s already untapped can’t be untapped again to pay the cost. A creature’s activated ability with the untap symbol in its activation cost can’t be activated unless the creature has been under its controller’s control continuously since their most recent turn began. See rule 302.6.

The player that controls Necrotic Plague at the time the enchanted creature is put into a graveyard is the player who controls the last ability (even though that creature's controller is the one who chooses its target). The first player returns Necrotic Plague to the battlefield under their control, regardless of whose graveyard it was put into. Note that the ability is mandatory; a target must be chosen if able, and Necrotic Plague must be returned to the battlefield if able.

Necrotic Plague grants the sacrifice ability to the creature it's enchanting. That means it triggers at the beginning of the upkeep of the creature's controller (not necessarily Necrotic Plague's controller).

If no legal target can be chosen for Necrotic Plague's last ability, it just remains in its owner's graveyard. Similarly, if the targeted creature becomes an illegal target by the time the ability resolves, the ability doesn't resolve and Necrotic Plague remains in its owner's graveyard.

Necrotic Plague's last ability triggers when the enchanted creature is put into a graveyard for any reason, not just when it's sacrificed due to the ability it grants to that creature. It triggers if it and the creature it's enchanting are both put into the graveyard at the same time, or if the creature it's enchanting is put into the graveyard but Necrotic Plague isn't. (In the second case, Necrotic Plague is then put into the graveyard as a state-based action.)