The easiest powers a witch can access is the spells. As for most powers available to the witch, the spells requires some sort of reagent or component to cast. But unlike the potions which can sometimes require numerous (and often rare) components, spells need only one.

What spells then are known to the coven?

Only the witches know, and they seldom tell outsiders. But much wailing and gnashing of teeth has been heard where the witches prowl, dealing out quick curses on those who fail to show proper respect!