A single player parkour map, but instead of jumps it's more running through obstacles, with a strict time limit. If you slow down or fail then you get eaten alive by spiders. There is no combat, not puzzles, just pure action.
There are plenty of checkpoints around the map, and you need to reach each one three times before it counts. It can be a little difficult at first, the learning curve is quite steep and mistakes are punished mercilessly, but through a little trial and error you should be able to figure out the best path through the map. You'll probably die a lot though, just warning you.
*NOW WITH 'EASY' MODE*
More info: http://www.minecraftforum.net/topic/1713418-147-spider-chase-run-for-your-life/