The speaker begins with an admission of failure: "We could not mend the great stone." A menhir is a tall, upright stone, usually ancient, and here it seems to be broken or damaged in some way. The word "we" suggests a group that tried and couldn't fix it. Then the speaker watches the king heal it with nothing more than "simple water." What defeated the group is solved quietly, and almost casually, by one person.
The last line turns somewhere unexpected. The king leaves the stone and goes "to the burial of dirt." The poem doesn't explain this, so it stays strange and open. Judging by the wording, it could be a funeral, or a kind of symbolic return to the earth. The speaker sounds humbled, maybe awed, and perhaps a little left behind. The mood is hushed and mythic, like a fragment of an old story, where healing and death sit side by side and nobody stops to explain.