Astral Flow is the Summoner's special job ability in Final Fantasy XI. When used, the player is allowed three minutes to use an Avatar's "Astral Flow Blood Pact". These are very powerful offensive Blood Pacts that can deal a great deal of damage, and affect enemies within an Area of Effect. In addition, the summoner's Perpetuation Cost is reduced to zero, and elemental spirits will have greatly increased casting speed. Astral Flow also grants the Summoner the ability to summon Alexander and Odin if they have forged pacts with them. Alexander and Odin may only be summoned while the Summoner has the effect of Astral Flow.

The amount of MP required to cast the AFBP depends on the Summoner's level (specifically, twice the Summoner's current level). If the Summoner does not have the required amount of MP, the avatar the AFBP cannot be used. No matter how much MP the Summoner has, it will be reduced to 0 upon a successful casting of an AFBP. With the proper items and/or equipment, a Summoner may use up to three AFBPs per Astral Flow use, making this ability useful in many special battles and Burning Circles.

Like other special job abilities, this ability may be used by certain Notorious Monsters of the appropriate job; however, Astral Flow behaves differently when monsters use it. Even more confusingly, there are multiple monster variants of Astral Flow used by different monsters. The most common variant, used by enemy summoners who keep a summoned elemental or no pet at all around at all times, will instantaneously summon a random avatar which will stick around just long enough to use its AFBP, then vanish. The other common variant, associated with the beastmen summoners in Dynamis who keep a summoned avatar out at all times, works more like the player version, but the AFBP is only used once. The third variation is sometimes seen with monsters that have multiple summoned elementals out at once, and it causes the elementals to assist the summoner against their current target and step up their spellcasting. The Vampyr Nosferatu has a unique version of Astral Flow where it will summon three or six monsters—which can be bats, undead hounds, or Fomor—which appear long enough to do a random special attack, then vanish. Absolute Virtue also has a unique Astral Flow where all his Wynavs will use a breath attack at the same time.