A Realm Reborn 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

Other permanents you control have ": Add one mana of any color."

Complete guide

A Realm Reborn is a powerful green enchantment that transforms your entire board state into a massive reservoir of mana. By granting every other permanent you control the ability to tap for any color, it ensures you will never be short on resources for your most expensive spells. Whether you control a massive army of creatures or a collection of artifacts and enchantments, your utility becomes limitless.

Universal Mana Generation

While A Realm Reborn is on the battlefield, every other permanent you control—including creatures, artifacts, enchantments, planeswalkers, and even lands—gains the activated ability to tap for one mana of any color. Note that this is a mana ability, meaning it does not use the stack and cannot be responded to. It provides incredible color fixing for multi-colored decks and turns non-creature permanents that usually sit idle into active mana producers.

The Importance of Summoning Sickness

Even though A Realm Reborn grants an ability to tap for mana, it does not remove the rules regarding summoning sickness for creatures. If a permanent is a creature, you cannot use its new mana-tapping ability the same turn it enters the battlefield unless it has Haste. However, non-creature permanents like artifacts or enchantments can be tapped for mana immediately upon resolution of A Realm Reborn or as soon as they are played.

Interactions with Other Effects

A Realm Reborn only grants the ability to other permanents. It does not grant the ability to itself, so you cannot tap the enchantment to pay for its own upkeep or other spells. Additionally, if a permanent already has a tapping ability, it now has two separate abilities you can choose from. You can only tap a permanent for one of these abilities at a time, as tapping is a cost that can only be paid once per turn cycle unless the permanent is untapped by another effect.

Strategic Timing

With a mana cost of six, A Realm Reborn is a late-game engine. You should aim to play it when you have a wide board presence to maximize the immediate mana return. Because it turns every utility enchantment or mana rock into a prismatic lens, it is particularly effective in decks that require specific colors for high-impact finishers.

Interaction examples

Doubling Your Resource Potential

Doubling Season

With both enchantments on the field, Doubling Season gains the ability to tap for any color of mana thanks to A Realm Reborn. Furthermore, if you use your newly abundant mana to cast a spell that creates tokens, Doubling Season will double the amount of tokens created. Each of those new tokens will also immediately have the ability to tap for mana (though they must wait a turn if they are creature tokens).

Fueling Combat Growth

Unnatural Growth

Unnatural Growth provides a powerful combat trigger, but it often requires a heavy green mana investment. With A Realm Reborn, Unnatural Growth itself can be tapped to provide one of the green pips needed for other spells. During the beginning of combat, Unnatural Growth doubles your creatures' stats, and those same creatures can be tapped for mana in your second main phase if they survived combat and did not tap to attack.

Guide written with AI from the official text and rulings. Official English text prevails.