Remember Realm of the Mad God, the browser MMO that was all the rage back in 2012 or so? It's still chugging along on Steam, receiving regular updates. The latest is a doozy, promising a weeks-long alien invasion event.
Commander Calibrik and his alien army will be roaming around in the mountains, and if players team up to take them on Calibrik will arrive in a UFO for a boss fight. When defeated it drops a bunch of sweet loot and also causes wormholes to open around the realm, each of which leads to "a gauntlet style dungeon where you have to fight off waves of enemies as they teleport in."
It's all in service of handing out exotic loot called Alien Gear to players, as well as Alien Cores that act as power-ups and increase in strength the more Alien Gear you have equipped.