Imagine Dragons are touring the U.S. this summer and fall, which means frontman Dan Reynolds has to go to a lot of dark places. The band is debuting songs from “Mercury,” a double album of brooding and moody meditations on death and human frailty. Reynolds must relive the pain at every show. Integrating the new songs into a seamless, fun night out for fans has also been a challenge, with the band needing to somehow fit hits like “Radioactive” and “Believer” with songs like “Wrecked,” which Reynolds wrote about losing his sister-in-law to cancer.