Rimmer Dall is a main character in The Heritage of Shannara tetralogy. He is the leader of the Shadowen, a group of magic users who will stop at nothing to possess all magic. He is also influential in the upper echelons of the Federation, serving, ironically, as First Seeker of the Federation's anti-magic force.
Rimmer Dall first appears to arrest Par and Coll Ohmsford for Par's use of the wishsong in Varfleet. He later appears to Par in the vault housing the Sword of Shannara in People's Park in Tyrsis. There, he revealed his Shadowen nature to Par and claimed that Par's command of innate magic made him a Shadowen as well. He dares Par to use the Sword's magic on him, and Par is shocked to find that nothing happens. The First Seeker then lets Par escape with the Sword, all the while planning to possess the young man's body and the magic of the wishsong within it. To accomplish this, Rimmer Dall tricks Par into thinking that he killed Coll, while imprisoning the real Coll in Southwatch. Rimmer Dall then tricks Coll into escaping Southwatch by donning a magic cloak that begins to turn him into a Shadowen.
While this is going on, Dall also schemes to bring down other members of the House of Shannara as well as others working to bring down the Shadowen. He leads a party of bestial Shadowen to Hearthstone, where he confronts and apparently kills Cogline and Rumor, leaving Walker Boh barely alive. He also dispatches the assassin Pe Ell to kill Quickening, daughter of the King of the Silver River. He later dispatches the Four Horsemen of the Apocalypse to dispose of Walker and sends a Shadowen named Tib Arne to capture Wren Elessedil and sabotage the Elves' defense of the Westland against Federation forces.
However, the First Seeker's focus remains on Par, whom he eventually manages to capture. Dall is defeated when Coll, Walker, and their allies storm Southwatch. Caught in a battle with Walker Boh, Dall is unable to take over Par's body, or stop Par from using the Sword of Shannara on the captured magic imprisoned beneath Southwatch. Once Par learns the truth of the magic's origin and the origin of the Shadowen, he uses his wishsong to break the chains holding the magic in place, and the freed magic travels all about the Four Lands, taking back the energy stolen from it from all the Shadowen and turning them all to dust—including Rimmer Dall.