Crabomination

Emerge from artifact {5}{B}{B} (You may cast this spell by sacrificing an artifact and paying the emerge cost reduced by that artifact's mana value.)
When Crabomination enters, target opponent exiles the top card of their library, a card at random from their graveyard, and a card at random from their hand. You may cast a spell from among cards exiled this way without paying its mana cost.

Modern Horizons 3

Emerge from artifact {5}{B}{B} (You may cast this spell by sacrificing an artifact and paying the emerge cost reduced by that artifact's mana value.)
When Crabomination enters the battlefield, target opponent exiles the top card of their library, a card at random from their graveyard, and a card at random from their hand. You may cast a spell from among cards exiled this way without paying its mana cost.

Emerge from artifact {5}{B}{B}
When Crabomination enters the battlefield, target opponent exiles the top card of their library, a card at random from their graveyard, and a card at random from their hand. You may cast a spell from among cards exiled this way without paying its mana cost.

Modern Horizons 3 Promos

An artifact's mana value is determined solely by the mana symbols printed in its upper right corner (unless that artifact is the back face of a double-faced card, is a melded permanent, or is copying something else; see below). If the mana cost includes {X}, X is 0. If it's a single-faced card with no mana symbols in its upper right corner (because it's an animated land, for example), its mana value is 0. Ignore any alternative costs or additional costs (such as kicker) that were paid as the artifact was cast.

Colored mana components of emerge costs can't be reduced with emerge.

Crabomination's triggered ability doesn't allow you to play land cards, and as such, any land cards exiled with Crabomination's triggered ability will remain in exile. If you choose not to cast any of the exiled nonland cards (either because you can't or don't want to), those cards will remain in exile as well.

Emerge from artifact is a variant of the emerge ability. It allows you to sacrifice an artifact rather than a creature, but otherwise functions identically to emerge.

If you cast a spell for another cost "without paying its mana cost," you can't choose to cast it for any alternative costs. You can, however, pay additional costs, such as kicker costs. If the spell has any mandatory additional costs, those must be paid to cast it.

Once you begin to cast a spell with emerge, no player may take actions until you're done. Notably, opponents can't try to remove the permanent you wish to sacrifice.

The artifact chosen to be sacrificed is still on the battlefield up through the time that you activate mana abilities. Its abilities may affect the spell's cost, be activated to generate mana, and so on. However, if it has an ability that triggers when a spell is cast, it will have been sacrificed before that ability can trigger.

The mana value of a creature spell with emerge isn't affected by whether its emerge cost is paid. For example, if you cast Crabomination for its emerge cost and sacrifice an artifact whose mana value is 3, Crabomination's mana value remains 6.

The mana value of the back face of a double-faced card is the mana value of its front face. The mana value of a melded permanent is the sum of the mana values of its front faces. An artifact that's a copy of either has a mana value of 0.

The spell you cast without paying its mana cost is cast during the resolution of Crabomination's triggered ability. Timing restrictions of that spell based on card type are ignored.

You choose whether or not to cast one of the exiled cards as Crabomination's triggered ability resolves. If you do, you do so as part of the resolution of that ability. You can't wait to cast it later in the turn. Timing restrictions based on the card's type are ignored.

You may sacrifice an artifact with a mana value of 0, such as a Food token, to cast Crabomination for its emerge cost. You'll just pay the full emerge cost with no reduction.

You may sacrifice an artifact with mana value greater than or equal to the emerge cost. If you do, you'll pay only the colored mana component of the emerge cost.