For Rune and Spirit Magic,Rivers of Sartar uses a lightly modified version of the RQ:G cult rules that underlay the supplements in the Cults of Runequest (CoR) line. So in order to explain these rules, and justify their existence, it is necessary to first explain how the official rules work.

Hopefully this will also prove useful to those who want to stick with the Rules As Written (RAW), or make their own preferred tweaks.

RQ:G RAW

A PC is born into a clan, which is normally part of a tribe, and located in a homeland. Default PCs are all initiates, so worship a god via a Rune Cult. This means they have a Rune Pool (RP), which initially has a size of 3. This can be used to cast any of the three Rune Spells they initially know.

Rune Cults can be complex multifaceted things, and so many have one or more:

• magical relationships (cult associations) with other Rune Cults, such as a jointly-run shrine or ceremony.
• Subcults representing particular traditions of worship, aspects or understandings of the deity. These describe the ways in which different cult temples tend to vary.
• Subordinate Cults representing other magical organisations run by the cult, such a shrine or ceremony to a minor deity or hero tended by priests of the cult.

The description of a Rune Cult is generally written up using either a short form (as in RQ:G)  or long form (as in CoR) template. Sub cults usually only have a short form writeup, noting only how they differ from the default. However, a long form writeup of a subcult is possible, for example Barntar in CoR.

At the time of character creation, a PC will normally have only dealt with one tradition of worship, and so has been taught by only one subcult. So the cult skills learnt in Step 6 of character creation are technically those of the subcult picked by the PC. In most currently-published cases, these are the same for all subcults. Again, Barntar is an exception.

Using skills has no associated costs, and using Spirit Magic only costs Magic Points, which refill automatically. But In order for a Rune Spell to be reusable, they have to have a way to refill their rune pool after casting it. This is done using the worship skill appropriate to a cult that is available to. This means that if they  are not initiated into that cult, the spell is 1-use. So it not only goes away after casting, but permanently drops their RP size by the value of the spell. This is presumably true whether it was initially learnt from  a cult they later left, or they acquired the spell by some non-standard means. Such means include being a reward for a favor to a cult, or succeeding at a heroquest.

A Rune Spell will be available to a Rune Cult if any of the following apply:

• if it listed as Special Rune Magic in the most recent write-up
• if it is named as the spell provided by an Associated Cult
• if it a spell available to any  Subcult or Subordinate Cult.

Note that available to does not necessarily mean easily available; if a particular subcult is only available in Kralorela, the character must travel there to learn it. It is entirely GM's discretion if such a trip can be assumed to have taken place during the PCs backstory. So there is no rule explicitly restricting Rune Spells taken during character generation to only those available to their subcult. However, they must take at least one, other they would not actually be members.

Initiation into multiple cults

After character creation, initiation into multiple cults is possible, even if the cults are not associated.

However, membership in multiple cults confers additional obligations in terms of donations of income and time, without any correspondingly increased status or reward.

For skills and spirit magic, common sense applies; this only changes what is available for training, and at what cost. 

Things get more complex where Rune Magic is concerned, and the results differ between the case where a Subcult or Rune Cult is joined. 

In either case, once any preconditions are met and tests of initiation passed, the deal is sealed by learning a single Rune Spell, at a cost of 1 POW. But if joining a new top level rune cult, then a new Rune Pool is needed, corresponding to the fact that the Worship skill is cult-specific. Such split Rune Pools are then tracked separately.

This means that, unlike in most previous  Gloranthan rulesets, in RQ:G the question of whether two deities with different names are or are not aspects of each other has to have a rule-defined answer. Given that, it is better it be defined in advance than debated during play.

More common than multiple cult membership is transferring between cults, often in a way corresponding to different stages of life. For example, the CoR writes up of Asrelia states that priestesses of Ernalda past childbearing age may retire from that role and automatically become a Priestess of Asrelia.

Rivers of Sartar House Rules

A few minor  changes to the RAW are recommended, whether or not you are using the rest of these rules.

• In the case of split Rune Pools, the CHA cap on the size of a Rune Pool applies to the sum of such split Rune Pools. This avoids multiple cult initiation being not just more flexible, but a stronger route to raw power than focusing on a single cult.
• When transferring between friendly or associated cults in a way approved by the new cults, access to spells available to both cults is retained. The new Rune Pool has a size based on the number of such spells known.
• When joining an associated or related cult, the new Worship skill starts not at zero, but at a fraction of the most similar worship skill known, in the same way that related languages work. 

These changes means that a Priestess of Ernalda who knows Absorption, Hide Wealth,  Command Snake, Summon Earth Elemental and Fertilize will, on transferring to Asrelia, start with a Rune Pool of size 5.  If they had a Worship[Ernalda] skill of 90%, they now have Worship[Asrelia] 60%. So they are immediately able to function as a priestess, without requiring a multiple-year retraining period late in life. Of course, they still have a lot to learn about the deeper mysteries of the cult.

As usual, their other Rune Magic such as Birthing and Bless Champion becomes 1-use. Most priestesses ensure they expend these spells before transferring cults, as afterwards casting them will permanently shrink their Rune Pool size. Others hold them in reserve for some emergency.

More relevant to all but the most long-running campaigns, this also provides more flexibility early in life. So a PC might change from Orlanth to Lhankhor Mhy with only a small loss of magic. A warrior might even convert from Humakt to Yanafil Tarnils; while they lose a lot, they do not lose everything.