She was betrayed by her fellow Order member, Olivia Rivers. As a bargaining chip in an attempt to join David Thorpe's raider gang, Olivia offered him intel on the location of an upcoming supply run by members of the Order in Summersville.
When Olivia found out Allison had been chosen for the Summersville supply run, she tailed her to ensure that the ambush was a success. However, as a veteran Mistress, Allison successfully defended herself, overwhelming and killing the majority of ambushing raiders.
In order to salvage her chances of entry into the gang, Olivia was forced to kill her fellow Mistress herself. Olivia brought Allison's body all the way back to the Manor, with her fellow Order members remaining none the wiser of what she had done.
Allison Long is mentioned only in Fallout 76.