The first preview test for the expansion is running from now through Friday, November 10th. It includes all three public spaces, the new Barovia wilderness area, seven dungeons, and the racial AP tome. However, the team informed players that the test will not be covering any raids, the other five dungeons, sentient weapons, favor rewards, and landscape-specific loot.
The expansion will take players to the horror-themed realm of Ravenloft for the first time, becoming the third (or fourth, depending on how you count it) Dungeons and Dragons campaign setting that DDO has used since its launch.
Source: Official forums. Thanks DDOCentral!