I actually like the word choice of trapped, because it’s more confining. Having escape routes is more hopeful, but I think the strength of the tension is the rapid pace and negative word choices like trapped.
While that is technically true, I like the pacing of the sentence without the comma to maintain the tension. My suggestion would be to either reword or ignore the rule.
The Blight changed everything, sweeping the globe, leaving humanity on the brink of extinction. The melior, with their immunity and fortified walls, stepped in to provide sanctuary. Now, the last of the human race are enslaved, but alive.
