Copyright (c) ? Fabian Maeser

This game is played on an empty square board (say 6x6):

TURN - At each turn, each player drops one stone on an empty cell. After the drop, all (orthogonal) adjacent empty cells are also occupied.
GOAL - The player who makes the last move, wins.
An example

After the marked drop stones, the orthogonal adjacent empty cells are also occupied.

Regio can also be played with stones of just one color. This game fits the Combinatorial Game Theory criteria:

Two players moving alternately.
Both players have full information about the game-state.
No chance.
The first player that cannot move loses the game.

Check this article studying Regio using CGT.