Blob

The Blob begins innocently enough as a mere individual playing a game of tag. As soon as she catches someone, she joins hands with him. Now he’s part of the Blob, too, and they both set out hand-in-hand in search of victims. Everyone the Blob catches (only the outside hand on either end of the Blob can snatch at players) joins hands with it and becomes part of the lengthening protoplasmic chain. And thus the insidious Blob keeps growing.
Unlike your run-of-the-mill mad scientist created Blobs, this one is not content merely to ooze along, seeking its prey. It gallops around the field, cornering stray runners and forcing them to join up. (You’ll have to agree on boundaries for this game; some people will go to any lengths to avoid meeting with an untimely end at the hands of the primordial slime).
Moreover, the Blob can split itself into parts and, with its superior communal intelligence, organize raiding parties on the lone few who have managed to escape. The thrilling climax occurs when there’s only one player left to put up a heroic last-ditch stand on behalf of humanity. But alas, there is no defense against the Blob, and humanity succumbs. (If that seems unfair, well, that’s the plot.)