The normal version of Chronicle X is a one-against-many game with a number of Hero Players competing against one Alien Overmind Player. As an alternative, introduced as a stretch goal in the Kickstarter campaign for Chronicle X in response to requests from many backers, a solo/co-operative mode was added as an expansion.
In the solo/co-operative mode, the Overmind Player is replaced by a deck of cards.
For those who do not have the co-operative mode Expansion but would like to try it before trying to get hold of it, a print-and-play version of it is available from Archon Studio's download page:
The print-and-play version only support using the Alien Types in the Core Box, but this should be enough to try it out.