In the aftermath of the massive war with the undead king, the Norns, a race of elves that followed the Storm King, have broken and are trying to make their way back to safety in their homeland of Nakkiga.
This forces the Norns to travel through Rimmersgard where Duke Isgrimnur and his men are determined to take down every last pale-skinned immortal they can and punish them for the many lives lost in the recently won war. One such group of Norns contains not only the body of the fallen General Ekisuno, but also the High Magister of the Order of Builders, Aarike, his second-in-command, Viyeki as well as a small collection of other builders and some from the Order of Sacrifice (the Norn's military branch). Unable to make it all the way through Rimmersgard, Aarike decides that they should take shelter in an old fort called Tangleroot Castle. This starts the first of the two main sieges found in The Heart of What Was Lost.
While Isgrimnur and his men, including his loyal follower, Sludig Two-Axes, surround the Norn's escape, we also learn of a new pair of characters, Porto and Endri. The two meet on the road during the aftermath of the war and both decide to head north to continue their soldiering by joining Isgrimnur's siege force. While one of the main reasons of showing this pair's journey is because one of them will be present in the next book, it also shows a different perspective of the sieges, the view of a foot soldier, not the men shouting out commands and making the big decisions.
The siege on Tangleroot Castle lasts for some time, but the Norns do eventually break out, thanks primarily to an unforeseen force of Norns led by Suno'ku, a famed general in the ranks of the Order of Sacrifice. With her help, Aarike, Viyeki and many others make their way safely into the mountain citadel of Nakkiga where the book's other major siege kicks off, and this time, the Rimmersmen have all of the immortals trapped in one place.
The Heart of What Was Lost spins a good story that not only seems to act as a good bridge between the two Osten Ard series, but could even be seen as a one-off. Speaking as someone who hasn't had the chance to read the original series, Williams shows just enough about the Osten Ard world to let the reader understand what is going on in this book and no real knowledge of the surrounding plots are necessary in order to enjoy this tale. The book does explain that first time readers of an Osten Ard novel should read an essay written in the back before starting off on The Heart of What Was Lost. This essay, written in the voice of an in-world character, goes a long way into describing the two immortal races, the pale-skinned Norns and their cousins, the Sithi. While the Sithi have only a small presence in this novel, understanding the Norns' relationship with them, as well as their relationship with the mortals, is a big help when diving into this book as your introduction to the bigger world.
Personally, I really enjoyed how well the novel showed the sieges from both perspectives, that of the forces outside of the walls, and the view of those trapped inside. For one, it shows the hardships of both positions, but it also gives the reader a deeper understanding of the history and culture of the Norns. Again, having not read the first books, I don't know if Williams had already established such a history for the enemy henchmen before, but I found it akin to learning more about the culture of the Orcs or Uruks in The Lord of the Rings or the Trollocs in The Wheel of Time, something generally not covered in epic fantasy.
The Heart of What Was Lost is a compelling story that anyone interested in the other Osten Ard novels should want to pick up. I know I will soon be reading The Witchwood Crown, and the original trilogy won't be far behind it, and The Heart of What Was Lost helped to prepare me for the world I'm about to dive into.