Kandredin is undoubtedly Diana Wynne Jones' most terrifying villain. He first appears in The Spellcoats through the actions of his magic, which poison The River in some way and cause a much larger than usual flood.
Meanwhile Kankredin is in a big black ship anchored near the mouth of the River. The ship is swathed in a dark gray cloud, and can't be seen directly. There is also a huge net across the mouth of the River, which is catching flying pinkish beings who struggle in the net, then slip into the water, where they are caught in baskets by Heathens in small boats.
On the ship there are a number of unhealthy, malicious-looking Heathen mages in long robes with words woven into them, saying things like "I tortured the beast" or "I sent the hidden death". The three youngest children in the family sail their boat through the cloud and past the net, and climb up onto the deck of the ship, where the mages descend on them when Hern opens one of the baskets to see the pinkish things inside. Duck recognizes them. "They're human souls," he says, and the other two realize he's right.
The mages bring them to Kandredin, who is sitting inside the cabin on the ship. He is old and evil-looking, with squiggly gray hair on each side of his face, heavy-lidded cold eyes, and a long robe woven from collar to hem with words.
As Hern challenges Kankredin, who can't quite figure out how they got past the net, Tanaqui finds herself reading the words on his robe and realizes that he is waiting to capture the soul of Gull, who he knows is the conduit to the souls behind the powers of the land.
As the story goes on, the children leave, thanks to the pipes that Tanamil made for Duck, and thanks to the fact that Kankredin has seen Duck's baby name woven into his robe, but cannot enchant him with his real name Mallard, which he doesn't know.
Although the children do manage to get away this time, Kandredin reappears in this story in several frightening ways, and then shows up again in the fourth and final book of the series. He is a really really bad villain.