From Breaking Worlds

When a mortal dies, their soul is taken by the Guides into the spirit world, and they become an ancestor spirit.

Because the ancestors were once human they are both easier to relate to than elemental spirits and more willing to give advice to living mortals. Another key difference is that it is almost unheard-of for one to consent to being bound to a living human. When summoned they appear marked by a black X on their foreheads.

An ancestor’s influence diminishes over time, but it also appears to depend on how well they are commemorated by the living. Celebrations, memorials, and offerings will maintain their power (and improve their mood), with the result that many families - especially within the Noble Houses - maintain elaborate shrines to departed family members. It is also generally held that mortals’ actions in life affect their standing after death, but exactly how is fiercely debated, and there are a great many schools of thought with their own particular views on how to live so as to benefit most in the afterlife.

In most dealings with the ancestors, the established custom is to call any ancestor who is not a blood relation “grandfather” or “grandmother”, or “aunt” or “uncle” if they died young. Even where this is not followed it is considered sensible to show plenty of respect, so as not to anger the spirits…

Since the Breaking, ancestor spirits have been observed returning to their bodies and reanimating them as revenants. This is obviously difficult for everyone but is a particularly thorny problem for priests. What do you do if you’re trying to venerate an ancestor who’s got back up and is walking around?