The cost reduction applies only to generic mana in the total cost of legendary creature spells you cast.

Serah Farron's last ability checks at the moment it would trigger to see if you control two or more other legendary creatures. If you don't, the ability won't trigger at all. If it does trigger, the ability will check again as it tries to resolve. If you don't control two or more other legendary creatures at that time, the ability won't resolve and none of its effects will happen.

The mana value of a nonmodal double-faced card is the mana value of its front face, no matter which face is up.

A nonmodal double-faced card enters with its front face up by default, unless a spell or ability instructs you to put it onto the battlefield transformed or allows you to cast it transformed, in which case it enters with its back face up.

In the Commander variant, a double-faced card's color identity is determined by the mana costs and mana symbols in the rules text of both faces combined. If either face has a color indicator or basic land type, those are also considered. For example, Cecil, Dark Knight's color identity is black and white, since its front face is black and its back face has a white color indicator.

Each face of a nonmodal double-faced card has its own set of characteristics: name, types, subtypes, abilities, and so on. While a nonmodal double-faced permanent is on the battlefield, consider only the characteristics of the face that's currently up. The other set of characteristics is ignored.

Each nonmodal double-faced card in this release is cast face up. In every zone other than the battlefield, consider only the characteristics of its front face. If it is on the battlefield, consider only the characteristics of the face that's up; the other face's characteristics are ignored.

To determine the total cost of a spell, start with the mana cost or alternative cost you're paying (such as a flashback cost), add any cost increases (such as kicker costs), then apply any cost reductions (such as that of Serah Farron or Crystallized Serah's first abilities). The mana value of the spell is determined by only its mana cost, no matter what the total cost to cast that spell was.

The back face of a nonmodal double-faced card usually has a color indicator that defines its color.

If you are instructed to put a card that isn't a double-faced card onto the battlefield transformed, it will not enter at all. In that case, it stays in the zone it was previously in. For example, if a single-faced card is a copy of Crystal Fragments, it will be exiled during the resolution of its second ability and remain in exile.

A token that is created as a copy of a double-faced permanent or a double-faced card in another zone is a double-faced token. It will have both the front face and back face of whatever object it's copying. If it's copying a double-faced permanent whose back face is up, the token will enter with its back face up. It can transform if instructed to do so.