Cogs

Introduction: This set of linked cogs makes for a battlefield where everything keeps moving and the combatants must ensure they pay as much attention to their surroundings as they do their opponents.

Setting/System: Setting could be anything; fantasy or sci-fi are the obvious ones, but there’s nothing that couldn’t be adapted to a modern game. System-wise, you’ll need to run it with a map and miniatures for positioning, but it doesn’t necessarily need a combat system as crunchy as D&D.

Description: These cogs are found in the belly of some giant machine. They sit in a horizontal formation, so can be walked on with relative ease, though the faster moving ones might be tricky to maintain balance on. Technically the cogs are moving at constant speeds, but for practical purposes, they move at set moments, as noted in the mechanics below.

Mechanics: Each size of cog has its own speed. The most practical way to run the combat would be to have the cogs on separate tiles that can be physically turned with all the combatants on them.
[Note: I am aware that the speeds below do not actually tally with the relative speeds that such cogs would have in reality.]
1) This giant cog turns very slowly, making a quarter turn clockwise at the end of every other round. Only half of the cog is accessible as the rest disappears into the larger machine. Anyone in area A at the end of the round will take damage as they are pushed against the moving (and sharp) mechanisms at B.
2) These medium cogs make a half turn anti-clockwise at the end of each round. Each of them has a low hanging support strut over one half of the cog; any character that would pass through the strut as a result of the cog’s rotation, must make a saving throw or be knocked prone (though they still end up in the same square either way).
3) This small cog makes two half turns each round; one at the end and one part way through the round (either roll initiative each round or assign it an initiative about halfway down the order, as appropriate to the system).
4) It is impossible to remain on this tiny cog as it spins too quickly. Passing across it requires a dexterity check (or similar); failure means that the character crosses successfully, but falls prone in the first square of the next cog and ends his movement.

Encounter: You could run any fairly standard combat with this set up, as the moving terrain provides plenty of interest. Melee enemies are a must; you don’t want everyone just standing on separate cogs and shooting each other. Flying enemies would also be an interesting challenge, with their ability to ignore the cogs’ movement, though this does make them powerful, so don’t have too many of them.