The Lich Lord opened three portals, to Rin, Lion's Arch and Thunderhead Keep. Each portal is guarded by three Portal Wraiths and destroying a portal is only done by destroying them. These portals were opened at the end of the Abaddon's Mouth mission and were destroyed at the end of the Hell's Precipice mission. (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.)