A Legendary Ocean

 Atlantis (which was also an ancient setting in the Waking the Dragons arc) is the name of a legendary island first mentioned in Plato's dialogues Timaeus and Critias. After a failed attempt to invade Athens, Atlantis sank into the ocean in a single day and night of misfortune.

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This area appears in the artworks of Warrior of Atlantis, Poseidra, the Atlantean Dragon, Legendary Atlantean Tridon, Plate Salvage and Edict of Atlantis.

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This was the first card to manipulate a monster's Level.

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This card is an upgraded form of Umi.

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Lemuria, the Forgotten City is also similar to this card in that it's also treated as Umi and affects WATER monster's Levels, but increases them instead.

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TCG Rulings

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Codarus: If you control "A Legendary Ocean" and "Forgotten Temple of the Deep" you only need to send 1 of them to the Graveyard to activate its effect.

Forgotten Temple of the Deep: If you control this card and "A Legendary Ocean" you can send either to the Graveyard to activate the effect of "Codarus."

Psychic Tuning: Calculate damage for “Psychic Tuning” based on the Level of the monster when it was Summoned (before any effects on the field are applied). (If you Special Summon a Level 5 WATER monster and “A Legendary Ocean” is active, you take 2000 damage for “Psychic Tuning”.)

Sea Dragon Lord Gishilnodon: “Sea Dragon Lord Gishilnodon” activates its effect when a monster is sent to the Graveyard that was Level 3 or lower on the field and Level 3 or lower in the Graveyard. If a Level 4 monster that is Level 3 because of “A Legendary Ocean” is sent to the Graveyard, the effect of “Sea Dragon Lord Gishilnodon” does not activate.

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OCG Rulings

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Codarus: You can also send to the Graveyard "A Legendary Ocean" or a "Forgotten Temple of the Deep" which has its effect active as a cost to activate the effect of "Codarus".: Effect Monster > Codarus

Codarus: If you control multiple cards like "A Legendary Ocean" and "Forgotten Temple of the Deep" that are all treated as "Umi", then you can activate the effect of "Codarus" or "Levia-Dragon - Daedalus" or "Ocean Dragon Lord - Neo-Daedalus" by only sending one of those cards to the Graveyard; you do not send all of them.: Must you send both "A Legendary Ocean" and "Forgotten Temple of the Deep" to the Graveyard to activate the effect of "Codarus"?

Warrior of Atlantis: "Warrior of Atlantis" can add "A Legendary Ocean" from your Deck to your hand, even though the name of "A Legendary Ocean" is treated as "Umi" while in the Deck.: Can "Warrior of Atlantis" add to your hand an "A Legendary Ocean" whose name is treated as "Umi"?

Warrior of Atlantis: Discarding "Warrior of Atlantis" from the hand to the Graveyard is a cost to activate its effect which adds "A Legendary Ocean" to your hand.: Effect Monster > Warrior of Atlantis

Blind Destruction: "Blind Destruction" looks at the monster's current Level. If "A Legendary Ocean" is active, you look at the monster's new Level to see if it will be destroyed.: For cards which refer to the monster's Level, do you apply effects?

Give and Take: "Give and Take" increases your monster's Level by the Level of the Summoned monster while the Summoned monster is on the field. If "A Legendary Ocean" is active and "Give and Take" targets a WATER monster that is Level 4 while in the Graveyard, the monster's Level is 3 while on the field, so you increase the Level of the other target by 3.: When the Level of the monster Special Summoned by "Give and Take" changes, what happens to the Level of your monster?

Montage Dragon: When Summoning "Montage Dragon", "A Legendary Ocean" has no effect when determining its Original ATK.: When sending monsters with altered Levels to the Graveyard to Special Summon "Montage Dragon", what does his ATK become?

Relieve Monster: If a monster's Level becomes 4 by the effect of something like "Cost Down" or "A Legendary Ocean", then you can use the effect of "Relieve Monster" to Special Summon it to the field.: With the effect of "Relieve Monster", can you Special Summon a monster whose Level is 4 because of some effect?

"Ritual Spell Cards": When using Ritual Spell Cards, "A Legendary Ocean" will not reduce the number of Levels that you need to Tribute. However, "A Legendary Ocean" will reduce the Level of the monsters being Tributed (so that you will have to Tribute more monsters to complete the Ritual Summon).

Sea Dragon Lord Gishilnodon: The effect of "Sea Dragon Lord Gishilnodon" will not activate if a monster is not Level 3 or lower while in the Graveyard, even if the monster is Level 3 or lower monster before being sent to the Graveyard.: Does the effect of "Sea Dragon Lord Gishilnodon" activate when a monster whose Level is reduced to 3 by "A Legendary Ocean" is sent to the Graveyard?
Example: "A Legendary Ocean" turns "Mother Grizzly" into a Level 3 monster. When it is sent to the Graveyard, the effect of "Sea Dragon Lord Gishilnodon" will not activate and its ATK will not become 3000 because "Mother Grizzly

Previously Official Rulings

On Being "Umi"

You can have 3 cards total in your Deck between "A Legendary Ocean" and "Umi", because this card’s name is treated as "Umi". So you cannot have 3 copies of "Umi" and 3 copies of "A Legendary Ocean" in your Deck.

The name of "A Legendary Ocean" is treated as "Umi" always and for all purposes. If "Imperial Order" is active, the card name is still treated as "Umi". If the opponent activates "Cursed Seal of the Forbidden Spell" targeting "Umi", you cannot activate "Umi" or "A Legendary Ocean" for the rest of the Duel.

When "A Legendary Ocean" is active, you may activate "Tornado Wall".

When "A Legendary Ocean" is active, "The Legendary Fisherman" on the field is unaffected by Spell Cards, so it does not receive the ATK/DEF bonus or have its Level reduced by 1. It still is treated as Level 4 in your hand, so you may Summon it without Tribute.

"A Legendary Ocean" does not have the effect of "Umi" that increases/decreases the ATK and DEF of specific monster Types.

In Other Rulings

Levia-Dragon - Daedalus: You can send "A Legendary Ocean" to the Graveyard to activate "Levia-Dragon - Daedalus" effect, but you can't send "Maiden of the Aqua".

Levia-Dragon - Daedalus: You can't send a face-down"A Legendary Ocean" to activate "Levia-Dragon - Daedalus" effect.

Prohibition: . "A Legendary Ocean" cannot be declared with "Prohibition", because its name is treated as "Umi".

Tornado Wall: "Tornado Wall" may be activated when"A Legendary Ocean" is face-up on the field.

On Decreasing Levels

When "A Legendary Ocean" is active, you may Summon or Set Level 5 WATER Monsters from your hand, since they are treated as being Level 4 monsters. If you Set a Level 5 WATER monster you do not have to show your opponent.

When the effect of "Cyber Jar" is activated and "A Legendary Ocean" is on the field, you cannot Special Summon Level 5 WATER monsters picked-up by the effect of "Cyber Jar". This is because the effect of "A Legendary Ocean" does not include Monster Cards in the Deck.

If "A Legendary Ocean" and "Gravity Bind" are active, you may attack with Level 4 WATER monsters (being treated as Level 3). If "A Legendary Ocean" is destroyed after an attack is declared, a replay occurs because the Level 4 WATER monster can no longer attack.

If "A Legendary Ocean" and "Infinite Dismissal" are active, and you Summon a Level 4 WATER monster (being treated as Level 3), it will be destroyed during the End Phase.

In Other Rulings

Ante: When "A Legendary Ocean" is active and you activate "Ante", when you check the Level, treat the Levels of WATER monsters in the hand as 1 lower for purposes of "Ante".

Cloudian - Nimbusman: If “Cloudian - Nimbusman's” Level is reduced to 4 or less by another card effect (“A Legendary Ocean”, “Level Conversion Lab,” etc.), you will not be able to Tribute any monsters when it is Normal Summoned because you do not offer Tributes to Normal Summon a Level 4 or lower monster.

Cost Down: While "A Legendary Ocean" and 1 "Cost Down" are active, your WATER monsters are reduced by 3 Stars.

D - Time: If you have "A Legendary Ocean" on the field and "Elemental Hero Bubbleman" is destroyed, you can activate "D - Time" and add a Level 4 or lower monster to your hand. If "Elemental Hero Bubbleman" is returned to the hand, then you can only add a Level 3 or lower monster to your hand because the effect of "A Legendary Ocean" still applies to "Elemental Hero Bubbleman" while he's in your hand.

Level Conversion Lab: If you activate "Level Conversion Lab" and change "Levia-Dragon - Daedalus" to a Level 5 monster, and "A Legendary Ocean" is on the field, then "Levia-Dragon - Daedalus" is Level 4.

Metamorphosis: "Metamorphosis" vs. "A Legendary Ocean": if you want to Special Summon a Level 6 Fusion Monster from your Fusion Deck, you have to Tribute a Level 7 WATER monster on the field because it is treated as a Level 6.

Montage Dragon: If you send a WATER monster from your hand to the Graveyard while "A Legendary Ocean" is active, the original Level of the monster is used and not the reduced Level.

Ninjitsu Art of Transformation: If "A Legendary Ocean" is active, your WATER monsters in your hand are lowered by 1 level so if you Special Summon from your hand using "Ninjitsu Art of Transformation", you can Summon at 1 level lower (effectively making it Level +4 if looking at the original Level).

Physical Double: If a Field Spell Card was active before resolution of "Physical Double" that applied to the targeted monster, it applies to the original monster and to the "Mirage Token". So if a Field Spell Card was increasing ATK by +500, the original would get +500 ATK and the "Mirage Token" would get an additional +500 ATK, for a +1000 ATK bonus total for the "Mirage Token" over the printed ATK of the original monster. Likewise, when "A Legendary Ocean" is active, and you activate "Physical Double" targeting a WATER monster, the "Mirage Token's" Level Stars are 2 fewer than the printed Level Stars of the original targeted monster.

The League of Uniform Nomenclature: A Normal Monster on the field that has its level lowered to 2 or less by"A Legendary Ocean"is a proper target for "The League of Uniform Nomenclature"

Thousand Energy: If you have "A Legendary Ocean" reducing Level 3's to Level 2, and the effect of "Thousand Energy" is applied, and then "A Legendary Ocean" is destroyed, the levels are returned to Level 3, the ATK/DEF bonus from "Thousand Energy" is lost, and those monsters are not destroyed during the End Phase.