Herigast, Erupting Nullkite

Emerge {6}{R}{R} (You may cast this spell by sacrificing a creature and paying the emerge cost reduced by that creature's mana value.)
When you cast this spell, you may exile your hand. If you do, draw three cards.
Flying
Each creature spell you cast has emerge. The emerge cost is equal to its mana cost.

Modern Horizons 3

Emerge {6}{R}{R}
When you cast this spell, you may exile your hand. If you do, draw three cards.
Flying
Each creature spell you cast has emerge. The emerge cost is equal to its mana cost.

Modern Horizons 3 Promos

You may sacrifice a creature 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.

A creature's mana value is determined solely by the mana symbols printed in its upper right corner (unless that creature 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 creature was cast.

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

Herigast's triggered ability will resolve before Herigast does. If Herigast is countered or otherwise leaves the stack in response to its triggered ability, the triggered ability will still resolve as normal.

If a spell has more than one emerge cost, you may choose to pay it for any one of those emerge costs.

If you cast a spell for another cost "rather than paying its mana cost", such as an emerge cost, you can't choose to cast it for any other 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.

If you sacrifice a creature with {X} in its mana cost, that X is 0.

Once you begin to cast a spell using an emerge ability granted by Herigast, losing control of Herigast won't affect it. You can finish casting it as normal. For example, you can choose to sacrifice Herigast as you choose to pay a creature spell's emerge cost from an emerge ability granted by Herigast itself.

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 creature 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 Herigast for its emerge cost and sacrifice a creature whose mana value is 3, Herigast's mana value remains 9.

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. A creature that's a copy of either has a mana value of 0.

You may choose to exile your hand even if you have no cards in hand. If you do, you'll still draw three cards.

You may sacrifice a creature with a mana value of 0, such as a token creature that's not a copy of another permanent, to cast a spell for its emerge cost. You'll just pay the full emerge cost with no reduction.