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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.

Any enters-the-battlefield abilities of the copied creature will trigger when Callidus Assassin enters the battlefield. You get to choose the order of those abilities and the triggered ability it has due to its copy effect. Any "as [this creature] enters the battlefield" or "[this creature] enters the battlefield with" abilities of the chosen creature will also work.

If the chosen creature is copying something else (for example, if the chosen creature is another Callidus Assassin), then your Callidus Assassin enters the battlefield as whatever the chosen creature copied.

You can choose not to copy anything. In that case, Callidus Assassin enters the battlefield as a 3/3 creature. Notably, it does not have "When this creature enters the battlefield, destroy up to one other target creature with the same name as this creature."

The triggered ability that Callidus Assassin has is part of its copy effect and is a copiable value that other effects may copy.

Callidus Assassin copies exactly what was printed on the original creature (unless that creature is copying something else or is a token; see below) and it has the triggered ability. It doesn't copy whether that creature is tapped or untapped, whether it has any counters on it or any Auras and Equipment attached to it, or any non-copy effects that have changed its power, toughness, types, color, or so on.

If the chosen creature is a token, Callidus Assassin copies the original characteristics of that token as stated by the effect that created the token. Callidus Assassin is not a token in this case.

If Callidus Assassin somehow enters the battlefield at the same time as another creature, Callidus Assassin can't become a copy of that creature. You may choose only a creature that's already on the battlefield.