Ancestor worship is almost universal at an early stage of development among sentient species. Shamanism is generally practiced at the same time by individuals with the gift but shaman generally deal with animal or nature spirits.
Most people engaged in ancestor worship have little power and their ancestor spirits had little when they were alive and so not much as spirits either. They can’t do much but will try to gift some luck bonuses to their descendents using the power gifted by their descendent’s prayers. An ancestor-worshiping player who conducts a prayer ritual once a week can have a small luck bonus token they can use at any time.
Some cultures practise an ancestor-worshiping version of shamanism/necromancy. Here the ancestor worship is more involved with each clan/house operating their own cult with dedicated tombs and gifted individuals within each house trained to become the equivalent of shaman/necromancers. These individuals have some power and make stronger spirits so their ancestors have more power also in a repeating cycle.
(Think the Dunmer in Morrowind.)
These ancestor shaman can summon ancestors to aid them in various ways. The first step is to create a focus like a spirit belt or spirit necklace which the summoned ancestor will inhabit and impart some bonus depending on their ability in life. A strong warrior ancestor might impart strength, an archer ancestor dexterity or archery skill, a hunter might impart tracking, a strong shaman ancestor might increase power.
(The ritual to summon an ancestor spirit generally takes place at night and the summoned spirit arrives before dawn.)
Initially an ancestor-shaman might learn how to make their spirit belt and summon one ancestor and later up to six, one at a time at first and later (after upgrading their spirit belt) up to three at once. The most experienced ancestor-shamans can also summon an actual spirit warrior to fight alongside them.
Ancestor-shaman cultures are often associated with necromancy by outsiders however generally such cultures will be extremely hostile to necromancers as ancestor-shaman cultures only summon *their own* ancestors and see the necromancy of other people’s ancestors as the ultimate blasphemy.
When specifying the ability or bonus imparted by a particular ancestor spirit to an NPC create a 2d6 table something like:
2-3: Hunter, tracking
4-5: Archer, Archery and DEX
6-8: Warrior, STR and END
9-10: Assassin, Stealth and AGI
11-12: Shaman, Pow, Magic Resist
The table will depend on the culture of the group and what skills, abilities they use most e.g. hunter might be the most common choice.