The Chaos Legion is the current theme or season for card packs in Splinterlands. Meaning, that any pack you buy right now is from the Chaos Legion theme. The symbol at the top of a card will tell you what theme it belongs to. There are seven in total. Just about any card you purchase right now, certainly all the ones on Splinterlands' marketplace, are from the Chaos Legion. Players have hoarded packs from previous themes, but good luck getting your hands on those.