Legendary/Legend rule

If a player controls two or more legendary permanents of the same name when state-based effects are checked, that player chooses one of those permanents and immediately puts the others into their owners' graveyards, without any player having an opportunity to respond. This does not destroy the other permanents, does not cause them to be sacrificed, and cannot be prevented by being indestructible or having regeneration.
==Description==
This version of the rule has been in effect since the release of Magic 2014.
Currently, only a handful of cards circumvent the "legend rule":
*<c>Mirror Gallery</c>, and <c>Sakashima of a Thousand Faces</c> cancel the rule entirely. 
*<c>Cadric, Soul Kindler</c> and <c>The Master, Multiplied</c> cancel the rule for tokens you control. <c>Sliver Gravemother</c> cancels the rule for Slivers you control.
*<c>Brothers Yamazaki</c> ignores the rule for itself as long as exactly two copies of the card are on the battlefield. The third will "kill" the two others.
*<c>Aeve, Progenitor Ooze</c>, <c>Jace, Cunning Castaway</c>, <c>Jace, Mirror Mage</c>, , and <c>Osgood, Operation Double</c> copy themselves and the resulting permanent isn't legendary, while thirteen cards can create copies of permanents with the same clause. <c>Ratadrabik of Urborg</c> makes nonlegendary copies of Legendaries that die, which is relevant if they return otherwise.
*<c>Sakashima the Impostor</c>, , <c>Lazav, Dimir Mastermind</c>, <c>Lazav, the Multifarious</c>, and <c>Sarkhan, Soul Aflame</c> can copy another legendary creature but keep their names, thus a player can have two legendary creatures with the same functionality.
==Older versions of the legend rule==
===From Legends to Champions of Kamigawa===
Originally, only one creature of the same name, with the creature type Legend, could be in play at the same time. For a while, they were even on the restricted list, meaning there could be only one creature of the same name in each deck. This was changed around the time of Ice Age.
Any person could play a Legend provided that that Legend wasn't already on the battlefield. If it was, that card was stuck in its owner's hand. They could cast it if they wanted to, but the newest one would immediately be put into the graveyard, so there was usually no incentive to do so.
This issue came to great prominence during the Masques block because Rebel decks centered around <c>Lin Sivvi, Defiant Hero</c> were dominant at the time. The card was so key to the deck that when two Rebel decks played one another, the first person to get Lin Sivvi out had an unfair advantage. Tom LaPille laid out the various disagreements about the rules change in his article.
===From Champions of Kamigawa to Magic 2014===
The second version of the rule checked to see if any other legendary permanent of the same name existed on the entire battlefield (regardless of the permanents' controllers) and sent all of those permanents (including the one which initiated the situation) to their owners' graveyards. In effect, each legendary permanent served two purposes: its original purpose and the removal of all instances of that permanent already on the battlefield.
===Rosewater version===
Mark Rosewater has stated multiple times that he considers legendary to be a mechanical downside that he would rather get rid of. If he was starting over he would make legendary a supertype with no rules baggage. He would create a keyword, called something like "unique", for things that needed for gameplay reasons to be restricted to having only one in play. The rest of R&D doesn't concur with Rosewater's idea, or feels it is too late for the change.
==Planeswalker uniqueness rule==
Although different, planeswalker cards used to have a similar rule: If a player controls two or more planeswalkers that share a planeswalker type, that player chooses one of them, and the rest are put into their owners’ graveyards. This was called the “planeswalker uniqueness rule.”
Before the above update, the original rule acted as the legend rule did from Champions to Magic 2014 until this was changed at the same time the legend rule was; it looked at the battlefield and didn't care which player's control they were under. The most recent copy remained while all others were sent to the graveyard. This allowed, for example, someone to play <c>Jace Beleren</c> as a removal spell for <c>Jace the Mind Sculptor</c> as they shared the same subtype "Jace". 
Starting with Ixalan, this rule was abandoned. All planeswalkers past, present, and future gained the supertype legendary and became subject to the "legend rule". Thus, if a player controls more than one legendary planeswalker with the same name, that player chooses one and puts the other into their owner's graveyard. This means for example that if you control <c>Jace, Unraveler of Secrets</c> and cast <c>Jace, Cunning Castaway</c>, both Jaces now can exist under your control.
The change was made to simplify gameplay.
==References==
Category:Magic rules