Based on a quote from the Shadow Priest artifact weapon:
I'm not sure we have figured out what "the Scouring" was, so I'm here to speculate.Xal'atath whispers: This land has seen so much upheaval. The Cataclysm, the Sundering, the Scouring. You really have no idea.
First, looking at the word scouring and what it means:
scour1
ˈskou(ə)r/Submit
verb
gerund or present participle: scouring
1.
clean or brighten the surface of (something) by rubbing it hard, typically with an abrasive or detergent.
"he scoured the bathtub"
synonyms: scrub, rub, clean, wash, cleanse, wipe; More
2.
archaic
administer a strong purgative to.
Must have been an event to systematically find someone/something and wipe it out of existence. Clearly this took place on Azeroth since the quote also references the Sundering and Cataclysm.
Old God related? Definitely, but at one point in history could the Old Gods have enough power and ability to carry out such a task? Perhaps during the initial reign of the Black Empire.
Personally I think this refers to the time of the Old Gods waging war and enslaving the Elemental Lords. It really doesn't have a feel to "scouring" the raging elementals off the planet so perhaps there is another event in history we do not know about yet?