Phantasy Star Online Episode III: C.A.R.D. Revolution
You can choose either side, the Arkz or Heroes, which both determines the abilities and cards available to you and the strategy decisions you must make in battle. The Heroes are the stronger of the two at first, with various weapons available to them, while the Arkz can summon creatures to their aid. Gameplay is similar to Magic: The Gathering or Yu-Gi-Oh, using the power and abilities of each card in turn to defeat the enemy.
Episode III can be played both offline and online, with the online system using a ranking system so you can compete against fair opponents.