The Khizra Khalasar is one of the three invading factions added in version 1.2 of the mod alongside House Targaryen and The Free Folk. It is lead by Khal Pono, and starts off at war with Volantis, immediately besieging and taking Selhorys.
NOTE: In version 2.1 The Khizra Khalasar can spawn as The Qevir Khalasar and will start taking over Qohor (needs confirmation)
Within the first 500 days of the game, you will get a message saying that Khal Pono's Khalasar has besieged Selhorys, and proclaimed himself ruler of the Khizra, which is the Dothraki word for 'marsh'. If unchecked by the player, it will eventually encompass all of the Volantene lands, after which it will start to act almost exactly like a normal faction, their next move unpredictable.