The Twelvefold Invocation of the Lord of Evening
Arcanum: Summon Darkfiend
Ritual Elements: Ritual Timing (The Conjunction, on the Longest Night; +10), Drawing Upon Shadow (Murder of twelve pure children; +15), Increased Ritual Time (1 hour; +4); Total Élan: 29.
Élan Traits: Binding Circle (5), Greater Aptitude (+5), Arcana Mastery, Command or Banish a Greater Demon in the Name of the Lord of Night (10 Arcana)
According to the notes, it is a Summon Darkfiend check DC 45 to bind the Blight Walker.