Here is the line describing Grendel's end:
Grendel fled, wounded, death-sick, under marshy hills to his joyless den; with that huge wound he knew for certain his life had ended, the sum of his days.
Kennings are relatively easy to spot - normally, they are hyphenated words which together create a new meaning. So, here, the kenning used in the description of Grendel's end is death-sick, which basically means that he is dying.