During the time of weakness right after the Guardians stopped the Sacrian Order's war, Kazeros and his army of Lost Ark Gold Demons invaded. Both the Guardians and the races of Arkesia were distracted, weakened, and unprepared for such a large-scale invasion. And, because of this, Kazeros managed to beat back the Guardians and Humans, create a new Red Moon in Arkesia that massively increased his power, and force back the Guardians (and even convert some of them to his side). Thankfully, Seven Heroes (including Luterra and some others that players meet throughout the endgame story quests) rose up and worked together with the Guardians over many battles to fight back.
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Eventually, this fight starts to wear down the heroes, but not before they come up with a plan to restore the pieces of the Primal Ark in order to seal Kazeros and push the Demons back to Petrania. They do so with Luterra controlling the restored Ark, and Kazeros' soul abandons his body in order to escape the seal, leaving a corpse behind that humanity seals beneath Mount Antares.
Once Kazeros was defeated, the Red Moon he created in Arkesia destroyed itself, causing its shards to rain down randomly across the globe. And, anyone who comes into contact with one of these and survives absorbing its power (without going mad in the process) becomes a Delain, a half-human half-demon.
Armen is obviously one of these, but players who skip the story likely don't know how he became one, or that cheap Lost Ark Gold the Sacrian Order exiled all the Delain to their own country of Feitan.