"Legend rule"

From MTG Salvation Wiki

The "legend rule" is the name for a rule that prevents two or more of the same legendary permanent from being in play at the same time. If a legendary permanent is in play, and one or more legendary permanents of the same name is put into play, all will go to their owners' graveyards when state-based effects are checked. This is not a destruction effect, and can't be prevented by regeneration or indestructibility.

From the Comprehensive Rules: 420.5e If two or more legendary permanents with the same name are in play, all are put into their owners’ graveyards. This is called the “legend rule.” If only one of those permanents is legendary, this rule doesn’t apply.

At this time, only two cards circumvent the "legend rule." Brothers Yamazaki allows up to two of them to be in play at the same time (though a third will send them all to the graveyard), and Mirror Gallery cancels out this rule.

[edit] Rulings

• If an effect asks a player to choose a creature type, Legend is not a valid option.
• Changing a legendary creature's type or subtypes doesn't stop it from being legendary.