In Shakespeare's Julius Caesar, Act 5, scene 3, Titinius uses a metaphor to lament Cassius' death; could you please explain this metaphor?

In Act Five, scene three, of Shakespeare's
Julius Caesar, Titinius discovers Cassius who has had his servant
kill him, believing all was lost in the battle with Antony and
Octavius.

When Titinius seems Cassius, he begins to mourn
the loss of his friend. His lament compares the setting sun with the ending of the life
of Cassius. As the red rays of the setting sun sink past the horizon to disappear into
the night, so too has Cassius—like the sun—sunk his red blood into the earth, and he has
passed into the night like the sun. Shakespeare extends the comparison stating that
Cassius is the sun of Rome, and it has set. As a day is over when the sun sets, the days
of Cassius and his friends and army are over as well. There is no light to lead or warm
them, only "clouds, dews, and danger."

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O setting sun,

As
in thy red rays thou dost sink to night, (65)

So in his red
blood Cassius' day is set,

The sun of Rome is set! Our day
is gone;

Clouds, dews, and dangers come; our deeds are
done!

Titinius is lamenting
the death of Cassius and all he stood for, even the hopes his army had, and that with
Cassius' passing, it foreshadows the defeat of the conspirators' army—or this is how
Titinius sees it before he, too, takes his own life out of his love for his dead
friend.