  We consider multi-player graph games with partial-observation and parity
objective. While the decision problem for three-player games with a coalition
of the first and second players against the third player is undecidable, we
present a decidability result for partial-observation games where the first and
third player are in a coalition against the second player, thus where the
second player is adversarial but weaker due to partial-observation. We
establish tight complexity bounds in the case where player 1 is less informed
than player 2, namely 2-EXPTIME-completeness for parity objectives. The
symmetric case of player 1 more informed than player 2 is much more
complicated, and we show that already in the case where player 1 has perfect
observation, memory of size non-elementary is necessary in general for
reachability objectives, and the problem is decidable for safety and
reachability objectives. Our results have tight connections with
partial-observation stochastic games for which we derive new complexity
results.
