It's an iconographic shorthand for Christ's battered body (from the 39 lashes and other torments before the crucifixion); those are bleeding wounds. Here you can see that some of the "spots" are trickling blood (from British Library MS Add. 37049 f.20, early 15th century).

Later medieval Christianity was obsessed with the Passion, and especially with Christ's suffering for human redemption. Late medieval art, especially in northwest (Germany, France, Low Countries, England) Europe, is just drenched in depicted blood. Although the engraving you've got there is later, it is rooted entirely in late medieval Harrowing of Hell iconography.