The Lemurian forces are trying to summon something enormous to overtake Millennium City, and high-level players will be pitted against the bulk of this army and its ominiously named Bleak Harbinger. Fail, and Millennium City falls. Succeed, and you might be able to pick up a Lemurian hovertank. Oh, and the city won't be destroyed. There are also new hoverbikes available from both the Drifter and the C-Store, helping ensure that heroes who want to take on the invasion will have good equipment to do so before it ends on March 27th.
[Thanks to matixzun for the tip!]