Witch Hut is an adventure map location, where a hero can learn a new Secondary Skill by visiting them. The hut teaches the skill to basic level. It does not teach the skill from basic to advanced or from advanced to expert level. In other words, the Witch Hut has no effect if the hero already possess the skill. By default the Secondary Skill is randomly chosen, but cannot be Leadership or Necromancy. The map-maker can choose which Secondary Skills (including Leadership and Necromancy) are eligible for the draw, or choose just one.
Before the first visit there is no way to know what skill the Witch Hut will teach. Also, the hut does not ask wheather the hero wants to learn a new skill or not, but teaches it immediately the hero enters the hut. Therefore it may be sensible to visit an unknown hut with a secondary hero to avoid unwanted Secondary Skills.
Witch Hut is one of four ways heroes can acquire Secondary Skills that are restricted from their class. For example, Barbarians cannot learn Water Magic when they gain levels or by visiting Universities. But visting a Witch Hut that teaches Water Magic will add it to their Secondary Skills. Other ways to learn restricted skills are, Seer's Huts, Scholars and events.