Grey Worm pulls Varys out of his chamber, and the savvy political operator knows it's all over for him. Tyrion tells him he was the one who tattled to Dany, before saying an emotional goodbye to the guy he just, you know, got the death sentence. In a solemn night ceremony attended by Jon Snow, Daenerys sentences Varys to death and then gives the order: "Dracarys."
It's a good thing she's finally decided to turn Drogon loose, but this probably isn't what Missandei had in mind. No, what Missandei had in mind is what happens over the course of the rest of the episode. Dany takes Drogon on the war path, destroying the Iron Fleet surprisingly easily -- almost as easily as Euron shot down Rhaegal.
From there, Daenerys and Drogon take on the scorpions, those giant, spear-firing crossbows, which likewise crumbled rather quickly. But the real madness comes when the battle is won and the surrender bells start ringing. That's when something snaps inside Dany and she decides to embrace her inner Targaryen and start burning innocent civilians alive. Once the massacre begins and the episode stays with the people on the ground, showing their horror and pain, it's easy to see the cost of wielding such strength. Daenerys finally used her superior firepower, but it has brutal consequences.