Throughout his book Grimsrud makes references to "Powers" or to "the Powers," which sounds much like an atonement theory known as Christus Victor, which was originally presented by Gustaf Aulen in 1940. Aulen calls Christus Victor as the classic view of atonement that emerges with Christianity itself.
Gustaf Aulen (born this day, May 15, 1879; died 1977) was the Lutheran bishop of Strangnas, Sweden, and the leading figure in a loosely-defined movement within twentieth-century theology called the Lundensian Theology. Wrote Christus Victor, published 1940
On page 20 Aulen defines Christus Victor, "is the idea of the Atonement as a Divine conflict and victory; Christ-Christus Victor-fights and triumphs over the evil powers of the world, the 'tyrants' under which mankind is in bondage and suffering, and in Him God reconciles the world to himself."
Drake Shelton's review of Christus Victor
Derek Flood: Penal Substitution vs Christus Victor
Atonement and Sacrifice: Doctrine and Worship - St. Augustine and the Fathers
What are principalities and powers?
Chapter 7 from The New Being by Paul Tillich: Principalities and Powers
Walter Wink