I finished Matthias Lehner’s quest chain up until “Knowledge is a Terrible Burden”. This is right after you take control of a horde siege engine and start blasting at a scourge army. Matthias Lehner sends you back to Thassarian onboard the Skybreaker. Now, Thassarian is supposed to give you the quest “Tirion’s Help” which leads into “Tirion’s Gambit” that (Spoiler alert) has you infiltrate the Cult of the Damned and find Arthas’ frozen heart and weaken him. Problem is, Thassarian does not offer this quest. After I turned in “Knowledge is a Terrible Burden” it just ends right there.
Is the continuation supposed to happen in a later phase? I thought I needed to do some other quest chain to unlock it, but I’ve done every quest in Icecrown except for the Valhalas arena.