When I started Immortal Throne, I got an error message that the CD key was invalid. This is easily fixed as follows:
1. Go to your Steam entry and right click on Titan Quest: Immortal Throne and then Manager → CD Keys. 2. Copy the CD Key. 3. Open up RegEdit. 4. Navigate to Computer\HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\Iron Lore\Titan Quest Immortal Throne 5. Right-click in the entires and select New → String Value. 6. Name the value CD Key. 7. Right-click on the CD Key and select Modify. 8. Paste in the CD Key from step 2. 9. Click OK.
The actual RegEdit entry is (CD Key is masked):
Windows Registry Editor Version 5.00
[HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\Iron Lore\Titan Quest Immortal Throne] "CD Key"="XXXX-XXXX-XXXX-XXXX-XXXX"
Note: It may be also located at HKEY_CLASSES_ROOT\VirtualStore\MACHINE\SOFTWARE\Wow6432Node\Iron Lore\Titan Quest Immortal Throne .
Ref: https://steamcommunity.com/app/4550/discussions/0/1696048426805084495/