Sounds like David Drake's Lord of the Isles. Garric or-Reise is the long-lost heir to the kingdom, Ilna os-Kenset is the weaver of magic, and Liane bos-Benliman's friend Nonnus the hermit is the wielder of the distinctive knife.
From what I recall they're entertaining, but lacking a bit in structure. Stuff just seems to happen.
The books center on the lives of four youths from a small village on one of the islands that make up the archipelago of the book's world, known as the Isles. In the first novel, the character Garric or-Reise discovers that he is, in fact, the direct descendant of the last king to rule the Isles, exactly a millennium ago. Then, as now, the power that fueled all magic in the world rose precipitously, allowing the use of spells so powerful that they shattered the cohesion of nations, driving them apart and in the process killing Garric's ancestor, Carrus, and sinking the island of Yole.
The novels deal with Garric's claim to the throne of the Isles, and his attempts to unite them once again. In the process, he and his companions are challenged by foes both physical and supernatural, which they must overcome. The basic outline of many of the novels deals with the characters being split apart, often taken to parallel universes or distant periods in time, by some hostile entity or group, which they must then defeat before, or in the process of, re-uniting.
Magic in the series is based on a now-dead language, the Old Script. Its characters are usually drawn using an athame (made of a variety of substances including iron, twigs and ivory) and the words spoken aloud. The introduction states that these are actual incantations from the Classical world.