One with the Multiverse — ruling 1

You must follow all normal timing rules for spells cast and lands played using the middle ability and must pay all costs for spells cast this way. To cast a spell this way, you may choose to pay any alternative costs or optional additional costs the spell may have (such as a kicker cost). You must pay any mandatory additional costs.

One with the Multiverse — ruling 2

While casting a spell without paying its mana cost using the last ability, you can't pay alternative costs, but you may choose to pay any optional additional costs the spell may have (such as a kicker cost). You still must pay any mandatory additional costs.

One with the Multiverse — ruling 3

While casting a card with the prototype ability using the last ability of One with the Multiverse, you can either choose to cast it normally or choose to cast it as a prototyped spell. In either case, you won't pay its mana cost.

One with the Multiverse — ruling 4

If One with the Multiverse leaves the battlefield and later returns to the battlefield (or you play a second one), it's considered a new object and you may cast a spell using its last ability regardless of whether or not you did so earlier this turn using the enchantment that was on the battlefield before.

One with the Multiverse — ruling 5

Similarly, if you control more than one One with the Multiverse (Two with the Multiverse, if you will), each one's last ability applies independently, and you may cast a spell without paying its mana cost once for each of them during each of your turns. In this case, you will need to track which enchantment's permission you are using to cast a spell this way (due to the possibility that one of them might leave the battlefield before you use its permission that turn).

One with the Multiverse : règles, fonctionnement et interactions (MTG)

One with the Multiverse 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

You may look at the top card of your library any time. You may play lands and cast spells from the top of your library. Once during each of your turns, you may cast a spell from your hand or the top of your library without paying its mana cost.

Complete guide

One with the Multiverse is a powerhouse enchantment that grants you total access to the top of your library and incredible mana efficiency. It transforms your library into an extended hand and allows you to cheat the mana system once every single turn. This card is a must-have for players who want to dominate the late game by casting massive spells for free.

Unlocking Your Library

The first two abilities of One with the Multiverse allow you to look at the top card of your library whenever you want, even if you don't have priority. More importantly, you can play lands and cast spells directly from the top of your library. When using this permission, you must follow all normal timing rules; for example, you can only play a land during your main phase when the stack is empty and you have a land play remaining. You are still responsible for paying all costs associated with these spells unless you use the third ability.

Casting for Free

Once during each of your turns, you can choose to cast a spell from your hand or the top of your library without paying its mana cost. If a spell has an X in its mana cost, you must choose 0 for the value of X when casting it for free. However, you can still choose to pay optional additional costs, such as Kicker. If the spell has mandatory additional costs, like discarding a card or sacrificing a creature, you must still pay those even if you aren't paying the mana cost.

Multiple Instances and Re-entry

One with the Multiverse tracks its 'once per turn' limit per permanent. if you control two copies of the enchantment, you can cast two free spells per turn—one for each copy. Similarly, if the enchantment leaves the battlefield and returns during the same turn, it is treated as a new object. Even if you already used the free spell from the original enchantment, the 'new' one allows you to cast another spell for free that same turn.

When to Play It

Because this enchantment costs eight mana, it is best played in decks that can ramp quickly or cheat enchantments into play. Once it hits the board, try to use the free spell ability immediately to recoup your investment, preferably on a high-impact spell that protects your board or draws more cards to keep your momentum going.

Interaction examples

Playing a Land from the Top

With One with the Multiverse on the battlefield, you look at the top card of your library and see a Forest. During your main phase, while the stack is empty, you may play that Forest just as if it were in your hand. This does not count as your 'once per turn' free spell because playing a land is not casting a spell.

Casting a Prototyped Machine

If the top card of your library is a creature card with the Prototype ability, you can use the free spell ability to cast it. You may choose to cast it either as its large, expensive version or its smaller Prototyped version. In both cases, the mana cost is reduced to zero by One with the Multiverse.

Managing Additional Costs

If you use the free spell ability to cast a spell that requires you to sacrifice a creature as an additional cost, you do not pay the mana (the blue, red, or white pips), but you MUST still sacrifice the creature to successfully cast the spell.

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

How it works : One with the Multiverse

Original English text — Wizards of the Coast / Scryfall

#1October 14, 2022· Wizards of the Coast
You must follow all normal timing rules for spells cast and lands played using the middle ability and must pay all costs for spells cast this way. To cast a spell this way, you may choose to pay any alternative costs or optional additional costs the spell may have (such as a kicker cost). You must pay any mandatory additional costs.
No other card is named in this ruling.
#2October 14, 2022· Wizards of the Coast
While casting a spell without paying its mana cost using the last ability, you can't pay alternative costs, but you may choose to pay any optional additional costs the spell may have (such as a kicker cost). You still must pay any mandatory additional costs.
No other card is named in this ruling.
#3October 14, 2022· Wizards of the Coast
While casting a card with the prototype ability using the last ability of One with the Multiverse, you can either choose to cast it normally or choose to cast it as a prototyped spell. In either case, you won't pay its mana cost.
No other card is named in this ruling.
#4October 14, 2022· Wizards of the Coast
If One with the Multiverse leaves the battlefield and later returns to the battlefield (or you play a second one), it's considered a new object and you may cast a spell using its last ability regardless of whether or not you did so earlier this turn using the enchantment that was on the battlefield before.
No other card is named in this ruling.
#5October 14, 2022· Wizards of the Coast
Similarly, if you control more than one One with the Multiverse (Two with the Multiverse, if you will), each one's last ability applies independently, and you may cast a spell without paying its mana cost once for each of them during each of your turns. In this case, you will need to track which enchantment's permission you are using to cast a spell this way (due to the possibility that one of them might leave the battlefield before you use its permission that turn).
No other card is named in this ruling.