Saga creatures have two sections to their text boxes. The first section, above the type line, contains their chapter abilities, and the second section, below the type line, contains any other abilities (or italicized flavor text). Any abilities in the latter section aren't chapter abilities and apply no matter how many lore counters are on the creature.
You can't choose to pay any additional costs for the copy. However, effects based on any additional costs that were paid for the original spell are copied as though those same costs were paid for the copy too.
The copy is created on the stack, so it's not "cast." Abilities that trigger when a player casts a spell won't trigger.
Magic: The Gathering · ™ & © Wizards of the Coast · Illus. Kevin Glint
Card data via Scryfall. Rarix is fan-made; not endorsed by the rights holders.
Removing lore counters won't cause a previous chapter ability to trigger. If lore counters are removed from a Saga, the appropriate chapter abilities will trigger again when the Saga receives more lore counters.
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.
If the spell has any targets, the copy will have the same targets unless you choose new ones. You may change any number of the targets, including all of them or none of them. The new targets must be legal.
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 chapter III ability to trigger, but chapters I and II won't trigger again.
As a Saga enters, its controller puts a lore counter on it. As your first 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.
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.
If the spell that's copied has an X whose value was determined as it was cast, the copy will have the same value of X.
Once the number of lore counters on a Saga is greater than or equal to the greatest number among its chapter abilities, 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.
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.
If the copied spell divides damage or distributes counters among a number of targets, the division and number of targets can't be changed. If you choose new targets, you must choose the same number of targets.
After Summon: G.F. Cerberus's second or third chapter ability resolves, the next instant or sorcery spell you cast that turn will be copied whether or not it has targets.
If the spell that's copied is modal (that is, it says "Choose one —" or the like), the copy will have the same mode or modes. You can't choose different ones.
Due to a rules change that takes effect with this release (see below for details), a Saga that somehow loses all of its chapter abilities will not be sacrificed as a state-based action. It will also not gain a lore counter at the beginning of each of its controller's first main phases.
