Thornwatch is a graphic novel adventure game for 2-5 players and a Judge. Players are manifestations of the Thornwatch: scarred veterans of the Eyrewood who now live as spirits summoned by the ancient ritual of tying a knot of thorns around a birch tree. The Thornwatch summoned to a particular tree do not necessarily know each other or the true nature of the problem they've been called to resolve; all they know is that they've answered the call, and they're bound to see it through.

Thornwatch bestows upon one player the role of the Judge, who enacts the will of the Judging Wood. Each other player plays a Thornwatch character: the Blade, Briarlock, Greenheart, Guard, Sage, or Warden. The game begins with the Judge selecting a storyboard in which the Thornwatch manifest before a tree adorned with a knot of thorny brambles, surrounded by conflict. If the players overcome this first challenge, the Judge offers choices that lead to new storyboards and further adventure.

The game takes place on one of many maps that display forests, swamps, and villages of the Eyrewood. The pawns lay flat, as if the heroes, enemies, and denizens of the forest are characters in a set of comic book panels.

The core engine is the momentum system. This system tracks turn order and monster health, encourages teamwork, and creates dynamic combat with priorities that change turn-by-turn and round-by-round.

The Thornwatch players each have decks of cards which give them skills and actions, as well as playmats and trait cards to show their unique outlook on the world. They roll custom dice including hero dice gained by manifesting their traits. Each session Thornwatch players have an opportunity to gain knots, which grant their wearer a bonus, regardless of which Thornwatch they choose to manifest in a particular session.