You should obtain this achievement fairly early on in a playthrough, probably from your first Vanguard members if you use them consistently.
Relics have a chance to be spawned when experienced heros die. My first relic was generated from an old age death, but apparently battle deaths can also trigger them. A good rule of thumb is that if your hero is experienced enough to have a nickname, they should be experienced enough to potentially generate a relic.
This achievement will pop at the same time as the notification that asks you to choose the relic's recipient. A relic can only be willed by a hero of the same bloodline, so having several family keeps should also speed up your chances of a relic appearing.