The Tomb of Sargeras (formerly known as the Temple of Elune) is the Burning Legion's portal into Azeroth and the former center of kaldorei worship in Suramar, sat on the Broken Shore, an island south of Suramar and the Legion's last foothold on the planet.
The Temple served as the night elves' place of worship until the Great Sundering, where it was sunk to the bottom of the sea. Approximately eight centuries ago, the Guardian Aegwynn buried the corpse of the dark titan Sargeras' avatar in the Tomb after defeating him. After many hundreds of years buried in the Tomb, the avatar slowly corrupted the former holy Temple with destructive fel energies. During the Second War, the warlock Gul'dan raised the Tomb back to the surface and attempted to reclaim its powers, but the demons guarding it slew him before he could do so. Illidan Stormrage, the first of the demon hunters, also visited the Tomb in search for the Eye of Sargeras during the Third War; while almost stopped by the Warden Maiev Shadowsong, Illidan made the Tomb collapse and Maiev had to escape.
The Gul'dan from alternate Draenor later reopened the Tomb once more, and successfully turned it into a portal for the Burning Legion to invade Azeroth via. The mortal forces of Azeroth are presently attempting to close off the Tomb.