Esper
Plane — Alara
Artifact spells cost 
less to cast.
Whenever chaos ensues, creatures you control that are white, blue, and/or black become artifacts in addition to their other types until end of turn. Then each artifact creature you control gains vigilance, menace, and lifelink until end of turn.
Bruce Brenneise
Rulings
If there are additional costs to cast an artifact spell, or if the cost to cast a spell is increased by an effect (such as the one created by Thalia, Guardian of Thraben’s ability), apply those increases before applying cost reductions.
(wotc • 2023-04-14)
That ability can’t reduce the amount of colored mana players pay for a spell. It reduces only the generic component of that cost.
(wotc • 2023-04-14)
Esper’s first ability doesn’t change the mana cost or mana value of any artifact spell. It changes only the total cost players pay.
(wotc • 2023-04-14)
The cost reduction can apply to alternative costs such as prototype costs.
(wotc • 2023-04-14)
The set of artifact creatures affected by the chaos ability is determined as that ability resolves. Artifact creatures you begin to control later in the turn and permanents that become artifact creatures later in the turn won’t get the bonuses.
(wotc • 2023-04-14)
If a spell you cast has {X} in its mana cost, you choose the value of X before calculating the spell’s total cost. For example, if an artifact spell’s mana cost is {X}{X}, you could choose 2 as the value of X and pay {3} to cast the spell.
(wotc • 2023-04-14)
Rulings
If there are additional costs to cast an artifact spell, or if the cost to cast a spell is increased by an effect (such as the one created by Thalia, Guardian of Thraben’s ability), apply those increases before applying cost reductions.
(wotc • 2023-04-14)
That ability can’t reduce the amount of colored mana players pay for a spell. It reduces only the generic component of that cost.
(wotc • 2023-04-14)
Esper’s first ability doesn’t change the mana cost or mana value of any artifact spell. It changes only the total cost players pay.
(wotc • 2023-04-14)
The cost reduction can apply to alternative costs such as prototype costs.
(wotc • 2023-04-14)
The set of artifact creatures affected by the chaos ability is determined as that ability resolves. Artifact creatures you begin to control later in the turn and permanents that become artifact creatures later in the turn won’t get the bonuses.
(wotc • 2023-04-14)
If a spell you cast has {X} in its mana cost, you choose the value of X before calculating the spell’s total cost. For example, if an artifact spell’s mana cost is {X}{X}, you could choose 2 as the value of X and pay {3} to cast the spell.
(wotc • 2023-04-14)