Titania, Gaea Incarnate — ruling 1

When a pair of cards are melded, the result is a single creature that's represented by two cards. If the melded creature dies, both cards are put into your graveyard. As it leaves the battlefield, both of those cards are turned face up again. If the cards are put on the top or bottom of your library, you choose their relative order.

Titania, Gaea Incarnate — ruling 2

One card in each pair of meld cards has an ability that instructs you to exile the two cards and meld them. If you control more than one object with one of those names, you select one object with that name to exile.

Titania, Gaea Incarnate — ruling 3

When two cards are exiled and melded, they each leave the battlefield, then return together as one new object with no relation to either of the objects that left the battlefield. Counters, Auras, Equipment, and other effects that affected those two cards don't affect the melded permanent.

Titania, Gaea Incarnate — ruling 4

Only two cards belonging to the same meld pair can be melded. Tokens, cards that aren't meld cards, or meld cards that don't form a meld pair can't be melded. If an effect instructs a player to meld cards that can't be melded, those cards remain in exile.

Titania, Gaea Incarnate — ruling 5

While a meld card is in any zone other than the battlefield, it has only the characteristics of its front face. The same is true while it's on the battlefield with its front face up.

Titania, Gaea Incarnate — ruling 6

While a melded permanent is on the battlefield, it has only the characteristics of its combined back face. Any effects that modify how the new object enters the battlefield will consider only the combined back face.

Titania, Gaea Incarnate — ruling 7

Note that the permanent represented by the combined back faces has a color indicator.

Titania, Gaea Incarnate — ruling 8

The mana value of a melded permanent is the sum of the mana values of its front faces. A creature that becomes a copy of a melded permanent has only the characteristics of that combined back face, and its mana value is 0.

Titania, Gaea Incarnate : règles, fonctionnement et interactions (MTG)

Titania, Gaea Incarnate is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Reach, vigilance, trample, haste Titania's power and toughness are each equal to the number of lands you control. When Titania enters, return all land cards from your graveyard to the battlefield tapped. : Put four +1/+1 counters on target land you control. It becomes a 0/0 Elemental creature with haste. It's still a land.

How it works : Titania, Gaea Incarnate

Original English text — Wizards of the Coast / Scryfall

#1October 14, 2022· Wizards of the Coast
When a pair of cards are melded, the result is a single creature that's represented by two cards. If the melded creature dies, both cards are put into your graveyard. As it leaves the battlefield, both of those cards are turned face up again. If the cards are put on the top or bottom of your library, you choose their relative order.
No other card is named in this ruling.
#2October 14, 2022· Wizards of the Coast
One card in each pair of meld cards has an ability that instructs you to exile the two cards and meld them. If you control more than one object with one of those names, you select one object with that name to exile.
No other card is named in this ruling.
#3October 14, 2022· Wizards of the Coast
When two cards are exiled and melded, they each leave the battlefield, then return together as one new object with no relation to either of the objects that left the battlefield. Counters, Auras, Equipment, and other effects that affected those two cards don't affect the melded permanent.
No other card is named in this ruling.
#4October 14, 2022· Wizards of the Coast
Only two cards belonging to the same meld pair can be melded. Tokens, cards that aren't meld cards, or meld cards that don't form a meld pair can't be melded. If an effect instructs a player to meld cards that can't be melded, those cards remain in exile.
No other card is named in this ruling.
#5October 14, 2022· Wizards of the Coast
While a meld card is in any zone other than the battlefield, it has only the characteristics of its front face. The same is true while it's on the battlefield with its front face up.
No other card is named in this ruling.
#6October 14, 2022· Wizards of the Coast
While a melded permanent is on the battlefield, it has only the characteristics of its combined back face. Any effects that modify how the new object enters the battlefield will consider only the combined back face.
No other card is named in this ruling.
#7October 14, 2022· Wizards of the Coast
Note that the permanent represented by the combined back faces has a color indicator.
No other card is named in this ruling.
#8October 14, 2022· Wizards of the Coast
The mana value of a melded permanent is the sum of the mana values of its front faces. A creature that becomes a copy of a melded permanent has only the characteristics of that combined back face, and its mana value is 0.
No other card is named in this ruling.
#9October 14, 2022· Wizards of the Coast
A player prompted to name a card may name the combined back face, and each player has the right to know that combined back face's characteristics at all times.
No other card is named in this ruling.
#10October 14, 2022· Wizards of the Coast
If an effect moves a melded permanent to a new zone and then affects "that card," it affects both cards.
No other card is named in this ruling.
#11October 14, 2022· Wizards of the Coast
In the Commander variant, a meld card's color identity is determined only by the mana costs and mana symbols in the rules text of its front face. No symbols or rules text of the permanent it melds into are considered.
No other card is named in this ruling.

Interactions with other cards

These cards' text mentions Titania, Gaea Incarnate.

Titania, Voice of Gaea

Reach Whenever one or more land cards are put into your graveyard from anywhere, you gain 2 life. At the beginning of your upkeep, if there are four or more land cards in your graveyard and you both own and control Titania, Voice of Gaea and a land named Argoth, Sanctum of Nature, exile them, then meld them into Titania, Gaea Incarnate.