The poetic device used in the phrase "stand stock-still" in Grendel is alliteration. Alliteration, is the literaty device which refers to the repetition of identical initial consonant sounds in successive or closely associated syllables. In this particulat phrase, the cluster "st" is repeated in three consecutive words. As a method of linking words for effect, alliteration is also known as head rhyme or initial rhyme.
There is one specialised form of alliteration called Symmetrical Alliteration.
