Shadows & Shapes

From the introduction:

Shadows & Shapes is an ultra-lightweight fantasy RPG. The rules are only a few pages long, and suited for one-shot games.

All conflicts in Shadows & Shapes are resolved with playing cards. The combat mechanics are based around a trick-taking card game, so if you've ever played Bridge, Hearts, or Oh Hell, you'll have no trouble picking up Shadows & Shapes.

The combat system has an unusual feel, since the players' decisions are highly constrained. Although strategy is important, there is often a single optimum strategy.

Collaboration is at the heart of Shadows & Shapes. If you set a foot wrong, it might be somebody else who pays the price.