The Andestine are Nobodies consumed by the dark hearts of the Heartless in Omniar. Xemnas turned into the first Andestine, Nullatrum. He then recruited 20 other Nobodies and drove them into the darkness of their Heartless's heart. They strive to bring death to those with hearts of light.
Nullatrum searched and searched for dark hearts, and brought them to their Nobodies. Nullatrum recruited a total of 20 other Andestines, making the Alignment of XXI. Possessed by darkness, Nullatrum murdered all of the Andestines' (that he recruited) complete beings. Though it is nearly impossible to kill all of the complete beings, they thrive to destroy Kingdom Hearts. Not take it, but delete it. Each member of the Alignment (excluding Nullatrum) has their own special type of Andestine creature.
Though somebody may not have a Nobody nor a Heartless, they can still have an Andestine that lingers in their shadow. The Andestine is in no physical form, but brings bad thoughts and feelings to the person. They lead their heart to darkness for it to turn into a heartless. Then the Nobody achieves the goal of getting a heart, but from the Heartless. The Andestine will then become a physical being.
To destroy Kingdom Hearts, the alignment must reach Kingdom Hearts. Since Kingdom Hearts is light, the Alignment reaches it through Twilight. They find a gate from Twilight town going to Kingdom Hearts, but they need a Keyblade of Light to unlock the gate. They search for a new Keyblade wielder, but until then they keep their base around the gate and in The Depths of Unillumination.
The Andestine Emblem features the Chinese word for corpse (尸) side by side, one facing backwards. This symbolizes how Andestine want to bring death to those with light hearts.