Runeforge Champion
Creature — Dwarf Warrior
When this creature enters, you may search your library and/or graveyard for a Rune card, reveal it, and put it into your hand. If you search your library this way, shuffle.
You may pay 
rather than pay the mana cost for Rune spells you cast.
Andrey Kuzinskiy
Kaldheim (KHM)
• #26Rulings
Runeforge Champion's last ability provides an alternative cost to cast Rune spells. You can't combine this with other alternative costs that may apply. In the unusual case that a Rune spell has additional costs, you can pay those in addition to the {1}. If those additional costs are mandatory, you must pay them in order to cast the spell.
(wotc • 2021-02-05)
Casting a Rune spell for an alternative cost doesn't change its mana cost or mana value.
(wotc • 2021-02-05)
Runeforge Champion's last ability doesn't change when you can cast a Rune spell.
(wotc • 2021-02-05)
Rulings
Runeforge Champion's last ability provides an alternative cost to cast Rune spells. You can't combine this with other alternative costs that may apply. In the unusual case that a Rune spell has additional costs, you can pay those in addition to the {1}. If those additional costs are mandatory, you must pay them in order to cast the spell.
(wotc • 2021-02-05)
Casting a Rune spell for an alternative cost doesn't change its mana cost or mana value.
(wotc • 2021-02-05)
Runeforge Champion's last ability doesn't change when you can cast a Rune spell.
(wotc • 2021-02-05)