The Burning Legion's Last Invasion

The Legion invaded in full, and the assault on the Broken Shore cost the factions dearly. In answer, Azeroth's champions claimed legendary artifact weapons and carried the fight from the Broken Isles all the way to Argus, the Legion's home world, where the imprisoned Pantheon finally struck back and bound Sargeras himself.

In his last act of spite before he was chained, Sargeras drove his enormous sword into Azeroth, stabbing the wounded world-soul through the desert of Silithus. The golden blood that welled up - Azerite - became the most sought-after substance on the planet, and the wound itself set the whole next era in motion.