As of this writing: The Dungeons & Dragons Online Menace of the Underdark DLC is on sale for $24.99 (50% off) on Steam as the "Daily Deal."
Greater tome of learning
1,000 Turbine Points to purchase additional quests, cosmetic pets, experience boosts, buffs, and more.
Start new characters at level 4.
The Phiarlan Carnival, Attack on Stormreach, The Path of Inspiration, and The Dreaming Dark adventure packs are included with this title.
Please note that this DLC requires the base game Dungeons & Dragons Online on Steam in order to play.