Alliteration is the repetion of a sound in two or more words in close succession. The effect it produces depends on the sound being repeated. In the first example a "p" sound is repeated: powerful, pain. This makes for a harsh tone, which follows the harshness of the action. The second example repeats an "h" sound: heavy, hall. It softens the mood. The last example is less clear. One could point to the t's in Herot, trembled, which give the verse a hard tone.
