The First Eruption — ruling 1

While resolving The First Eruption's final chapter ability, you must sacrifice one Mountain if able. You can't sacrifice multiple Mountains to deal more damage.

The First Eruption — ruling 2

As a Saga enters the battlefield, its controller puts a lore counter on it. As your precombat main phase begins (immediately after your draw step), you put another lore counter on each Saga you control. Putting a lore counter on a Saga in either of these ways doesn't use the stack.

The First Eruption — ruling 3

Each symbol on the left of a Saga's text box represents a chapter ability. A chapter ability is a triggered ability that triggers when a lore counter that is put on the Saga causes the number of lore counters on the Saga to become equal to or greater than the ability's chapter number. Chapter abilities are put onto the stack and may be responded to.

The First Eruption — ruling 4

A chapter ability doesn't trigger if a lore counter is put on a Saga that already had a number of lore counters greater than or equal to that chapter's number. For example, the third lore counter put on a Saga causes the III chapter ability to trigger, but I and II won't trigger again.

The First Eruption — ruling 5

Once a chapter ability has triggered, the ability on the stack won't be affected if the Saga gains or loses counters, or if it leaves the battlefield.

The First Eruption — ruling 6

If multiple chapter abilities trigger at the same time, their controller puts them on the stack in any order. If any of them require targets, those targets are chosen as you put the abilities on the stack, before any of those abilities resolve.

The First Eruption — ruling 7

If counters are removed from a Saga, the appropriate chapter abilities will trigger again when the Saga receives lore counters. Removing lore counters won't cause a previous chapter ability to trigger.

The First Eruption — ruling 8

Once the number of lore counters on a Saga is greater than or equal to the greatest number among its chapter abilities—in the Dominaria set, this is always three—the Saga's controller sacrifices it as soon as its chapter ability has left the stack, most likely by resolving or being countered. This state-based action doesn't use the stack.

The First Eruption : règles, fonctionnement et interactions (MTG)

The First Eruption 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

(As this Saga enters and after your draw step, add a lore counter. Sacrifice after III.) I — This Saga deals 1 damage to each creature without flying. II — Add . III — Sacrifice a Mountain. If you do, this Saga deals 3 damage to each creature.

How it works : The First Eruption

Original English text — Wizards of the Coast / Scryfall

#1April 27, 2018· Wizards of the Coast
While resolving The First Eruption's final chapter ability, you must sacrifice one Mountain if able. You can't sacrifice multiple Mountains to deal more damage.
No other card is named in this ruling.
#2April 27, 2018· Wizards of the Coast
As a Saga enters the battlefield, its controller puts a lore counter on it. As your precombat main phase begins (immediately after your draw step), you put another lore counter on each Saga you control. Putting a lore counter on a Saga in either of these ways doesn't use the stack.
No other card is named in this ruling.
#3April 27, 2018· Wizards of the Coast
Each symbol on the left of a Saga's text box represents a chapter ability. A chapter ability is a triggered ability that triggers when a lore counter that is put on the Saga causes the number of lore counters on the Saga to become equal to or greater than the ability's chapter number. Chapter abilities are put onto the stack and may be responded to.
No other card is named in this ruling.
#4April 27, 2018· Wizards of the Coast
A chapter ability doesn't trigger if a lore counter is put on a Saga that already had a number of lore counters greater than or equal to that chapter's number. For example, the third lore counter put on a Saga causes the III chapter ability to trigger, but I and II won't trigger again.
No other card is named in this ruling.
#5April 27, 2018· Wizards of the Coast
Once a chapter ability has triggered, the ability on the stack won't be affected if the Saga gains or loses counters, or if it leaves the battlefield.
No other card is named in this ruling.
#6April 27, 2018· Wizards of the Coast
If multiple chapter abilities trigger at the same time, their controller puts them on the stack in any order. If any of them require targets, those targets are chosen as you put the abilities on the stack, before any of those abilities resolve.
No other card is named in this ruling.
#7April 27, 2018· Wizards of the Coast
If counters are removed from a Saga, the appropriate chapter abilities will trigger again when the Saga receives lore counters. Removing lore counters won't cause a previous chapter ability to trigger.
No other card is named in this ruling.
#8April 27, 2018· Wizards of the Coast
Once the number of lore counters on a Saga is greater than or equal to the greatest number among its chapter abilities—in the Dominaria set, this is always three—the Saga's controller sacrifices it as soon as its chapter ability has left the stack, most likely by resolving or being countered. This state-based action doesn't use the stack.
No other card is named in this ruling.