The Broken Isles are a set of islands that were once part of Kalimdor before the Great Sundering. After the Sundering, some of them were sunken to the bottom of the ocean northeast of the Maelstrom. One of them, Thal'dranath, also known as the Broken Shore, was later risen above the surface of the waters by Gul'dan during the Second War. It houses the Tomb of Sargeras.
The current Broken Isles are the Broken Shore, Suramar, Stormheim, Highmountain, Val'sharah, Azsuna, and the Eye of Azshara. After the alternate Draenor Campaign, Gul'dan from the alternate universe was sent to Azeroth to pave the way for a Burning Legion invasion of the world. He traveled to the Tomb of Sargeras and broke seals left by the highborne during the War of the Ancients, as well as seals left by the Guardian Aegwynn hundreds of years before, allowing a massive portal to open that begun the Burning Legion's largest invasion of Azeroth yet. The Alliance, Horde, and Argent Crusade soon launched a massive assault at the Broken Shore, but ultimately failed. During the assault, Vol'jin, the Warchief of the Horde, was mortally wounded, and Varian Wrynn, High King of the Alliance, was killed by Gul'dan. Tirion Fordring, leader of the Argent Crusade, was also severely wounded and captured by the Legion. Later, it was discovered the Pillars of Creation were on the Broken Isles. Dalaran was teleported there, and the Alliance and Horde soon journeyed there as well.