Answer:
Hi! I wish we had more information about where is this excerpt from, who is the author, etc. in order to make a better interpretation of it. Anyways, I'll try to help you by giving you the best answer possible with the facts we have here.
Explanation:
The task is asking for an explanation on the matter, so I would say, after carefully reading these lines, that the matter here is that this man, the captain of "a bawbling vessel" was different from who he used to be, according to the speaker. He was "darker", something had changed him, some situation that we don't know of, that is not described in the excerpt changed him for good, a "loss" that "cried fame and honour on him".
It is very difficult to understand what happened exactly, but we can see that this man is damaged and has become a dark soul.
Hope this helps!