Warning: The following text contains spoilers relating to the plot of the Prophecies Campaign.
The Titan Portals are portals opened by the Lich Lord on Hell's Precipice in the Ring of Fire Islands. These portals would have allowed the Lich Lord to send his army of Titans into the major cities of Tyria.
(Note: although the Lich says "Lion's Arch", "Thunderhead Keep", and "Rin", one of the portals looks like it leads to the Henge of Denravi. The Henge is attacked during the Titan Quests, so it makes sense that the Lich may have planned to send some titans there as well.)