From the designer's website:

Rombo is a boardless strategy game in which players add pieces to a steadily growing cluster in an effort to cut off enemy areas.

Rules

Two players, White and Black, each have 24 pieces of their colour called rombos. Each rombo has twelve identical faces.

Start: The game starts with two White and two Black rombos joined together in a cluster.

Play: Each turn the current player must add a rombo of their colour to the cluster. The new rombo must exactly meet at least two faces of existing rombos. It is not allowed to form closed holes which would make the cluster hollow.

Aim: A player wins by splitting the visible enemy faces into two or more disjoint sets. If neither player achieves this before the pieces run out then the game is a draw.