Mox Opal

[Maxpayne03]

On my first turn, i have {Memnite} and two {Mox Opal}; could I play memnite and two opal, tap both opals before the state based action to remove one legendary artifact?

[Sardok]

No.

The legendary rule does not use the stack. Thus you cannot respond to it.

[Prplprince]

That's the issue. I you have 2 artifacts and mox you could float the mana then cast another mox

[Sardok]

That's the issue. I you have 2 artifacts and mox you could float the mana then cast another mox

He doesn't have 2 artifacts and a mox though; he has a mox and a memnite. The mox can't trigger to tap for mana, and any new mox would be put in the graveyard due to the legend rule before anything can happen.

[FustyDavorite]

704.5k: If a player controls two or more legendary permanents with the same name, that player chooses one of them, and the rest are put into their owners' graveyards. This is called the "legend rule."

This action is state based, meaning that this would happen whenever the next player would recurve priority, or before anyone would have the chance to respond. Casting 2 moxes with a memnite out means you sac a mod before you get the opportunity to tap for mana.

[Ekann1]

The first state based action check is right after the mox resolves before people get priority.