#She is saying light em' up.
She knows she is about to die, and is basically telling Daenerys to "Burn them all" as the Mad King might say.

https://www.youtube.com/watch?v=u8K9XuPrXko

>Dracarys is clearly meant for Dany. Missandei knows that her life is over, and she is saying, you know, "Light them up."

In addition, she herself later confirmed this in an interview with elle.com.

>(NATHALIE) She's normally quite calm and stoic. Her yelling is not really something we've seen before. I wanted to make the most of this moment and make sure that it meant something. In a way it was her own war cry, to her friend and her queen. It's so funny because she'd told Grey Worm earlier, "My people are peaceful." But in her last breath she's like, "Burn them all! Burn these fuckers to the ground."

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