Starnheim Courser
Creature — Pegasus
Flying
Artifact and enchantment spells you cast cost 
less to cast.
Every worthy warrior who arrives in Starnheim is gifted a mount befitting their heroism, a mighty pegasus raised and trained by the valkyries.
Andrew Mar
Kaldheim (KHM)
• #32Rulings
To determine the total cost of a spell, start with the mana cost or alternative cost you're paying, add any cost increases, then apply any cost reductions (such as that of Starnheim Courser). The mana value of the spell is determined only by its mana cost, no matter what the total cost to cast the spell was.
(wotc • 2021-02-05)
The cost reduction applies only to generic mana in the costs of artifact and enchantment spells you cast. It can't reduce requirements of specific colors of mana.
(wotc • 2021-02-05)
Rulings
To determine the total cost of a spell, start with the mana cost or alternative cost you're paying, add any cost increases, then apply any cost reductions (such as that of Starnheim Courser). The mana value of the spell is determined only by its mana cost, no matter what the total cost to cast the spell was.
(wotc • 2021-02-05)
The cost reduction applies only to generic mana in the costs of artifact and enchantment spells you cast. It can't reduce requirements of specific colors of mana.
(wotc • 2021-02-05)