Meaning of a line of the poem "Slave's Dream"

A little more context would help.

"At night he heard the lion roar,
And the hyena scream,
And the river-horse, as he crushed the reeds
Beside some hidden stream;
And it passed, like a glorious roll of drums,
Through the triumph of his dream."
-The Slave's Dream,
by H.W. Longfellow

As you know in the poem the slave (once a king and man of great personal and political power) is dreaming of his past life. The dream becomes more significant as the poet creates the slaves final journey. He revisits his family and effectively says goodbye o his wife and children, his rides out over his beautiful kingdom and sees it again, and feels the love he had. Lastly at the climax he is in the wildest part and hears the very heart of nature , this audio part is important as it builds from the roar to the scream and creates tension. The sound becomes the wave upon which the spirit of the slave is able to leave this life. Longfellow uses the simile of the drum roll because it resonates and excites, it always signals something important is to come (a magic trick, or even an execution!) The phonics of the dr sound and the r of roll create a real ly magestic atmosphere...and then we see the final triumph, he has done it! He has used his dream and the power of his spirit journey to escape his slave life.
Hope this helps.