The Kolder first appear in Witch World.
Appearance: Kolder look like men that Simon knew from Earth, implying they have pale skin. Their eye color is not mentioned but they are described as large, they have flattened features, broad cheekbones, small noses with a low bridge, a small & narrow chin, and wear caps which block any showing of hair - if they have any hair. With some Kolder, metal helmets appear to be wired directly into their brains. Attempted removal of the helmets often kills the Kolder wearing it.
Life Span: The life span of a Kolder is unknown. They probably live at least as long as an Earth human (though with the technology they've shown they probably live a good deal longer).
Power: Kolder do not possess the Power per se. They can use a form of body and mind manipulation but they need their machines to do it. They seem to use forcefields like a form of telekinesis to immobilize & manipulate people and drugs, surgery & implants to turn people into their mindless slaves.
Other: Once occupying the areas of Yle and Gorm as well as their overseas island base, the Kolder seem to be hyper-technological in nature. They came through a Gate of their own making which was eventually destroyed as was their entire race. They left the world scarred and their strange technology as legacy. This technology has yet to be fully explored. None of the areas the Kolder once possessed have yet been inhabited again.
The Kolder are a mystery. They seem to have no qualms about turning people into zombies, but neither Dalesfolk nor Alizonders seem to ever be in their hordes. This might be partly explained, at least with the Dalesfolk, in the fact they only turned their attention to them later (though if their island base was really as close as maps show it to High Hallack it's still unclear why they couldn't have reached for the Dales first). When Simon Tregarth is captured and ends up on Gorm he does notice zombified people of different races, some he'd never seen before or heard of. Still, none of those match later descriptions of Dalesfolk. The majority of the Kolder's zombie army were of course from Gorm.
Instead of giving Alizon their weapons to directly attack Estcarp they instead have them attack the Dales. Even in the Web of the Witch World their motives for reopening their Gate - to get more men or more equipment - only leads to disaster as the skeleton-men from their own past come to attack them. Galkur's calling, possibly coupled with Witch manipulation on a more subtle level, is probably what kept them from striking more effectively against Estcarp.
The location of the Kolder island base is unclear. It is somewhere to the south in the ocean. Maps generally place it close to High Hallack, but this may not actually be the case.
Simon went to investigate some islands with suspected Kolder activities. These islands obviously had a Gate that lead to the machine world. Why this activity occurred was never explained, nor whether the Gate was closed behind Simon (he couldn't get back though it, but that doesn't mean it was closed permanently).