Coen has three trees: Swordmastery, Witchcraft and Vampirism. Two of them work whenever you like. Witchcraft does not, and the reason is worth understanding rather than memorising, because it explains several things that otherwise look like bugs.
Why the form blocks the school
A hex is not spoken. It is cut into the sorcerer's own body, and the wound is part of the working. When Coen turns, his wounds and scars close almost instantly.
So the vrakhir form heals the cut before the hex can take hold, and Witchcraft becomes impossible. The game says it plainly in both its tips and its manual: the magic needs a wound that stays open. It is also why the vrakhiri as a species have no sorcerers of their own.
Twenty-two perks that only exist before sunset
Witchcraft carries twenty-two perks, and every one of them is a daytime tool. If you build into the tree and then play the valley at night, you have spent skill points on an empty half of the clock.
That is not an argument against the tree. It is an argument for deciding early whether your run is a day run or a night run, and for reading the day and night equipment presets as part of the same decision.
Two abilities you might not expect to be Witchcraft
Compel Soul is a Witchcraft ability, so questioning the dead is daytime work. Several quests depend on it, and one of them is the Razkovniche chain, so a night-heavy run can stall on a corpse it cannot interrogate.
Travel splits along the same line. Mercurial Fervour is the human form's speed power and sits in Witchcraft. Shapeshift is the vrakhir equivalent and sits in Vampirism. Learn only one and half your days are slow.
Hexes are paid for in health
Both Witchcraft and Vampirism abilities cost health on top of activation charges. Swordmastery does not. So the two supernatural trees are the ones that can kill you through use, and health management is part of using them rather than a consequence of being hit.
Gear answers this directly. The health cost of a Witchcraft or Vampiric ability can be cut by up to 8%, on two separate stat lines. If you lean on either tree, those lines are worth more than a damage roll.
What is still not known
Each ability's effect, its rank count, its tier and its prerequisites are all confirmed. The numbers behind each individual rank are not.
The per-rank curve is not something the game states anywhere we can read it, so rather than print a guess, those lines are left out. A full build planner waits on that one gap and nothing else.