When the sword of Eava lies with the fated king,
And vengeance stolen his unfinished blood,
Then shall war ignite at confederate wellspring,
The sins of the father called unto the son.
This is the prophecy that Xaltra
relays to Astervitch
at the beginning of the prequel.
Prophecy Meaning (Spoiler)
When the Sword of Eava lies with the fated king
The 'Sword of Eava' refers to Astervitch—Astervitch is blessed by the gods with the gift of an arcanic reserve—while the 'fated king' is Evander—this relates to both Evander becoming king of Beourjen at the end of the series, as well as his comparisons to Andrei Duxtour. The line thus says 'lies with' to mean both the consummation of their relationship as well as Ast's loyalty to Evander.
And vengeance stolen his unfinished blood
In the second book, Evander finishes taking down the Torniets by killing Lady Torniet and her unborn child—Evander's child—as revenge for everything they put him and Astervitch through. This act virtually ends the Balassar bloodline: Evander is the last viable heir in the family and he makes the decision near the end of the book not to sire anymore children.
Then shall war ignite at confederate wellspring
'Confederate wellspring' obviously refers to Beourjen City—it's both the capital of the confederacy and revolves around the Obsidian River. The third book chronicles the beginnings of the revolution and uprisings here.
The sins of the father called unto the son
This line comes into play in a number of ways, namely Evander and Ryven dealing with their 'fathers'' prior decisions. But it also refers to what kind of a society Dmitri will be ruling due to Evander's choices throughout the series; he's fated to become the king of a confederacy that's essentially been torn down and reconstructed by his father through the revolution.