。
Blood trickled smoothly down the back of his neck.
Literal
His nape [object-を] blood [subject-が] smoothly ran-down.
首筋 (くびすじ, 'back of the neck, nape') is specific body-part vocabulary. すっと is a mimetic for smooth, quiet, effortless movement. 伝った (つたった, past of 伝う, 'to run along a surface, to trickle down') is the precise verb for liquid following the contour of a surface. The sentence is cinematically vivid — every word contributes to the sensory image.