The official website explains:
"Legends of Norrath is an online-only trading card game, based on the richly-detailed sword-and-sorcery world of Norrath from the massively multiplayer online role-playing games EverQuest and EverQuest II. Playable from within the MMOs and separately, Oathbound, the first set for Legends of Norrath features over 375 digital cards and is available in both a 55-card digital starter deck and 15-card digital booster packs. Play as one of many Oathbound avatars and decide your own fate as you choose between the paths of good or evil.
EverQuest® and EverQuest®II Interactivity: Catered specifically for the EverQuest and EverQuest II gamer, Legends of Norrath has been specially designed to work both with and within your existing EQ and EQII games. Launch LON from either game or individually via the stand alone client.