Monster Invasion

Unlike other events, this occurs in towns. GMs will summon hordes of powerful monster right in town, causing lots of death and destruction. These don't drop special event items or rewards, but still drop the ordinary items they would in the wild. The event ends when every monster is killed. Typically these are powerful bosses, like the Jr. Balrog, King Slime, Zombie/Orange Mushmom, Crimson Balrogs, and the like. Sometimes, the top bosses, like Horntail, Zakum, and Grandpa Gunboss are summoned in towns or free markets, requiring all of the high levels in the world to work together and take them out. Occasionally, the players get overpowered and the GMs must step in to aid them. These are the rarest and most sought-after of all events.

In the months of June and July 2007, GlobalMS had weekly summonings on Fridays, with a regular pattern. Perion was invaded by King Slimes and smaller slimes, Henesys was invaded by Zombie Mushmoms and many smaller mushrooms, Ellinia by pigs and boars, Kerning City by all 4 species of evil eyes, Sleepywood by MT-09 and aliens of Omega Sector, and finally Rombots and Block golems in Amoria. Then, the event heads over to Ludibrium's Path of Time, where massive numbers of Crimson Balrogs were summoned. GM's would revive players so that they could continue fighting.
However, these attacks sometimes can prove to be a more troublesome lag than bosses like Zakum, as the limit to the number of people in the map is unlimited, with a massive number of monsters and dead players. If you are planning on fighting in these invasions, you need to have a very good connection, or you may accidentally die due to the tendency of lag causing people to be unable to see their status bars clearly.

http://strategywiki.org/wiki/MapleStory/Quests/GM_Events