The running theme throughout the series is summed up in the prologue:

The world of Erador is a world quite unlike ours. The inhabitants, at first glance, appear to be stuck in some sort of Dark Age. They fight with swords and axes, and are ruled entirely by various monarchies. But delve a little deeper, and you find that Numea is buzzing with science, especially in the field of genetics. They may still be wearing medieval attire, but they already know about DNA and genetic engineering. And they've managed to find a certain gene which doesn't exist in our world. It is called the Eresten gene, after the first person to be found carrying it. The gene has eight forms, only half of which are active. The four active forms draw energy from air, earth, water and fire, allowing the carrier to use magic of that element. They also have moderate healing powers. But that's not what the story is about. I just thought you might like some background information. The people of Erador are divided into Clans. Each Clan has a specific purpose. For instance, there is a Clan of blacksmiths and a Clan of healers, a Clan of poisoners and a Clan of assassins. The Clan that makes up the monarchy is called Avalin. Almost all of its Clanmembers are corrupt by the time our story starts. Vokaya, the Clan of assassins, works alongside Avalin in exchange for protection from the law. That way, when somebody realises the monarchy is corrupt, the assassins can dispose of them before it becomes a problem. At least, this is the theory. What Avalin did not reckon on was the somebody being from Vokaya. His name is Kashti Vokaya, and he despises his Clan. So much so, in fact, that he went out of his way to make them disown him. But one person can make a difference, and before Avalin knows it, Kashti has managed to spark a war against the monarchy...
Men killing men. Friends betraying friends. War for the sake of peace. These events echo throughout history, and they make the Daggers of Time. Impossibilities, caused by history repeating. A child with a father younger than they are. A girl advising the King over 100 years before she will be born. Impossibilities that tear the very fabric of time, and threaten to consume the entire universe.
A stitch in time saves nine. A Dagger in time has the potential to destroy thousands.