Trilithon is a connection game with a symmetric winning condition for a hexagonal hex board. It has been described somewhat wrongly (as Obtuse Y) in Cameron Browne's book Connection Games.
The correct rules are: The six sides of the hexboard are labeled as three adjacent pairs. A pair of sides is said to be completed by a player, if both sides within the pair and a third side not in that pair are connected by an unbroken chain of stones belonging to that player. The player who completes two pairs wins. Only one player can win, and there are no draws possible.
The usual rules for connection games apply. Players take turns placing one stone of their color on any empty space, and the first move can be swapped according to the pie rule. Corners belong to both edges.