Frost Moon

Frostmoon is a new event, added in patch 1.2.2, that can be triggered in hardmode. It has 15 standard waves as well as 5 extra bonus waves. It's a Christmas-y variation of the original Pumpkin Moon, but the enemies are slightly tougher. Therefore, one could conclude that is is the most difficult event to get through in Terraria.

During this event, the moon changes color to look like the face of a snowman.

The Frost Moon is triggered by using a Naughty Present.