。
Throwing knives were plunged deep into the men's necks.
Literal
Throwing-use [possessive-の] knives [subject-が] men [possessive-の] nape [to-に] deeply thrust-stuck-[resultative-ていた].
投擲用 'for throwing' — 投擲 'throwing/hurling' + 用 'for use in.' 深々と 'deeply' is an adverb with emphatic reduplication. 突き刺さっていた 'were thrust/stuck in' is a compound verb (突く 'to thrust' + 刺さる 'to be stuck in') in the past resultative. Visceral combat narration.