As originally described in this blog post:

Hnefatafl is an ancient Norse board game that predates Chess by several hundred years. heXentafl is a modern variant played on hexagonal boards. In particular, this version can be played on a NXS board using NXS pieces (more on that later).

heXentafl is played on a 4x4 hex grid.  The King sits on the central space, the throne, surrounded by three defenders.  Six attackers sit on the outer corners of the grid.

The attackers must capture the King in order to win. The defenders must escort the King to any of the six corners in order to win. The defenders usually move first, but either works.

All pieces except the King move in a straight line, as many spaces as they like to an unoccupied space (like a rook in Chess).  Only one piece at a time can occupy a space. A piece cannot jump over another piece.

The King can move into any vacant adjacent space, moving one space at a time (similar to the king in Chess).

Pieces are captured by surrounding them on two sides.  In the image below, white moves into position to surround the King and capture it, winning the game.

The corners present an interesting challenge, as they can't be surrounded in the normal manner. Pieces can be captured on a corner by surrounding them in the manner shown below. Here black moves into position to capture the white piece.

You cannot "capture" your own piece by moving between two enemy pieces. In the image below, the black piece is not captured when it moves between the two white pieces.

The Throne

The central space of the board is the throne.  Only the King may occupy the throne.  When the King is on the throne, he is harder to capture. He must be surrounded on three sides as shown below.

heXentafl can obviously be played on a NXS board. You can either use chess pieces, or use NXS pieces. If you use NXS pieces, select a piece to be the King (a different color will help identify it), and then use any other pieces to represent the pawns. Just remember that the normal move and capture rules / markings for NXS do not apply.