Titan Quest: Immortal Throne adds an entirely new act to the end of the original Titan Quest, a new mastery called Dream, and a significant number of minor gameplay tweaks/additions and UI improvements.
One of the most significant improvements added to the gameplay is a warehouse used to store extra items not on the player character. The premise is that a traveling caravan has taken notice of the player's exploits and wishes to help by providing space. This feature also includes a secondary area for transferring items between save fills such that if you find a perfect item for another one of you characters you can send it to them. The caravaner will offer upgrades to the size of his storage area, but at a steep cost.
The option to turn off auto-loot pick-up has also been added. You can now hold either the Z or X key (by default) in order to show loot items on the screen that have been dropped by chests or monsters. The Z key shows up every item dropped, while the X key shows up only yellow items or greater (including potions and gold)
Artifacts were also added, which are an extra piece of equipment that the player can wear in addition to those from the previous game. These are not dropped by enemies or chests but are instead created by the player through the use of formula (which are dropped with regular loot) by combining relics and charms. These recipes will tell you what you need to build them, and once you have all the required items they can be combined by a special merchant called an Enchanter for a price. These usually add fairly large stat increases and come in the form of lesser or greater artifacts. Enchanters have the secondary ability to separate relics/charms from items for a price and at the expense of either the relic/charm or the item.
Also added to the game are magic scrolls. The scrolls act as one-time use spells available to any class with extremely powerful effects ranging from elemental attacks to offensive/defensive player buffs. Scrolls are typically only available from merchants at a high price and are sometimes called for in artifact formula.
Immortal Throne also raises the level cap for player characters from 65 in the unexpanded game to 75. Levels beyond 65 continue to grant the player 3 skill points and 2 ability points each just as before.
New Mastery: Dream
The new Dream mastery is the only true melee-magic hybrid class in the game. The mastery was designed to work equally well complimenting any of the other melee or magic masteries in the Game. Dream characters have access to a variety of spell and melee skills that warp the laws of reality and fold space, affecting both the body and mind of their enemies. Some of their skills include putting enemies to sleep, throwing waves of electrically charged folds of space into crowds of enemies, and vanishing from reality and reappearing nearby with a powerful melee attack. Dream characters also have access to the Nightmare pet who, though not as individually as powerful as many other pets, has the ability to significantly buff the powers of all other pets on its team. To help Dream characters balance these disparate skills, points spent in the Dream Mastery bar grant bonuses to both Strength and Intelligence.
For Immortal Throne, Iron lore added an entirely new area - Hades. It features a long and difficult quest line where you must find the entrance to Hades, learn why spirits are being denied the right to be judged by the Three, battle alongside fallen heroes of Greek legend, and eventually encounter Hades himself. Along the way, there have been a significant variety of new monsters of all kinds added to the game. Iron Lore estimated the length of this expansion at about ten additional hours of gameplay.
A number of easter eggs were added to Titan Quest via the Immortal Throne expansion. The largest was a secret cave that could be found on the beach near the beginning of the fourth act. The cave had a locked door bearing an entry that would only open if the player was carrying an otherwise useless item called The Overlord. The Overlord was a small turtle-shaped object (a nod to the number of oddly placed turtles that could be found throughout the entire game) that could only be dropped by the final boss, Hades. Once the player gains entry to the cave they would be faced by a wide variety of powerful monsters that looked like creatures from every act of the game but typically had very different, or vastly exaggerated, powers. Amusingly, each of these creatures was named after members of the development team. Other smaller easter eggs included small scrolls marked by the symbol of a turtle which could be found on rare occasions in normal loot drops. These scrolls had writing on them that varied from simple jokes, such as a letter from a styr in the beastman army to his loved ones, to serious poetry.
Minimum: Windows® 2000 or XP, 1.8 Ghz Intel Pentium IV or equivalent or AMD Athlon XP or equivalent, 512 MB RAM, 5 GB free hard drive space, 64 MB NVIDIA GeForce 3 or equivalent or ATI Radeon 8500 series with Pixel Shader 1.1 support or equivalent, DirectX® 9.0c compatible 16-bit sound card, Keyboard, Mouse
Recommended: Windows XP, 3.0 Ghz Intel Pentium IV or equivalent, 1 GB RAM, 128 MB NVIDIA GeForce 6800 series or ATI Radeon X800 series or equivalent, Soundblaster X-Fi series sound card.