Killian, Ink Duelist
Legendary Creature — Human Warlock
Lifelink
Menace (This creature can't be blocked except by two or more creatures.)
Spells you cast that target a creature cost 
less to cast.
"When your father is Dean Embrose, second best really isn't an option."
Ryan Pancoast
Rulings
Killian can reduce alternative costs. For example, if you choose to pay the {1}{B} alternative cost for Baleful Mastery while targeting a creature, that cost would be reduced to {B}.
(wotc • 2021-04-16)
If a spell has additional costs, apply those increases before applying cost reductions.
(wotc • 2021-04-16)
Killian, Ink Duelist can’t reduce the colored mana requirement of a spell’s mana cost.
(wotc • 2021-04-16)
Rulings
Killian can reduce alternative costs. For example, if you choose to pay the {1}{B} alternative cost for Baleful Mastery while targeting a creature, that cost would be reduced to {B}.
(wotc • 2021-04-16)
If a spell has additional costs, apply those increases before applying cost reductions.
(wotc • 2021-04-16)
Killian, Ink Duelist can’t reduce the colored mana requirement of a spell’s mana cost.
(wotc • 2021-04-16)