The slave achieves his dream. How could this be true? What effect did this poem have on people when slavery was legal? “The Slave's Dream” by Henry Wadsworth Longfellow
If the slave is dead, you can assume he is in a heaven. Many belief systems have defined heaven as complete happiness as it is for you yourself and your needs, wants and desires. If he is in heaven, he has his queen, his children, his kingdom, and all of his pleasures.
Likewise, being dead, he is now free from the control of his master. That dream for freedom has been realized.
This author wrote a series of anti-slavery poems. I believe there were 8 total. He was also considered one of The Fireside Poets. These guys work was likely read by families and discussed. Most of his work was pretty feel-good stuff. An author like him could slip something a little cotroversial every now and then and likely make people think. Conquering slavery took a long time, but I would argue that many people contributed to its expulsion. Longfellow gave his contribution.
I don't know what effect the poem "would" have on people because that depends a lot on whether the people supported slavery in the first place. I can say what Wordsworth probably intended.
He intended for people to see that the slaves that were being used so brutally were people too. He wanted them to see that the slaves came from a place where (he imagined) they were free. He wanted the people to see that the slaves had dreams and hopes and aspirations.
The dream came true because the slave died and, presumably, went to Heaven where all things are possible.