surprise inheritance

An unexpected bequest from someone's estate, typically from a distant relative, forgotten acquaintance, or someone whose wealth was unknown to the beneficiary.

The phrase often appears in fiction as a plot device that suddenly transforms a character's circumstances: She quit her job after receiving a surprise inheritance from a great-aunt she'd never met.

In real life, surprise inheritances are less dramatic but do occur when estranged family members die or when people are named in wills without prior knowledge.