Not really a GM event, but for EQ's 5th anniversary (and I guess Halloween 2004) at night a skeleton army would spawn in Butcherblock Mountains, East Commonlands, The Feerrott, Qeynos Hills, Steamfont Mountains, and Toxxulia Forest. The commanders dropped some decent stuff for lower levels.
http://everquest.allakhazam.com/db/item.html?item=27344
http://everquest.allakhazam.com/db/npc.html?id=14811