The speaker is already lying in a grave, and he is speaking to someone who will lie there next. He doesn't sound frightened or bitter. He treats the grave almost like a room or a bed he is handing on, something you "may have after me," whether by accident or by some angel's choosing. That small phrase leaves open whether death is random or arranged, and the poem doesn't pick.
The last lines turn the grave into language. It is "like a long last word / I could not say," and so the speaker must "be" it. In other words, what he couldn't put into speech, he becomes simply by being dead. The final word, "be," is quiet but heavy, since it names existence just as the speaker has stopped living.
The mood is calm, hushed and a little strange. There is a gentle intimacy in speaking to a stranger across time, and an acceptance that every life ends up as something left for others.