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The Caligoi we mainly built for the pure pleasure of world-building, but if anyone wants to use it as a setting for a role-playing game, here are notes on how to apply the Generic Universal Role-Playing System (GURPS, (c) Steve Jackson Games) with it.
Magic in the Caligoi follows the GURPS Magic rules, with certain exceptions. It is at normal mana level; only persons with magical aptitude can cast any spells at all. However, the GURPS Magical Aptitude advantage is not available to everyone in its normal form. The Caligoi instead use Humours, magical advantages and disadvantages, some of which are partly or wholly equivalent to GURPS Magical Aptitude. However, only characters of certain races can buy these advantages. Some races can buy a limited magical aptitude humour which allows them to learn only certain kinds of spells. (We use the spell Colleges from GURPS Magic to simplify such classifications.) In such cases, the advantage costs less than does general Magical Aptitude - usually 5 or 10 points per level. For example, Blirthibo mages can only learn water spells, and Pliv mages can only learn earth and enchantment spells; it costs them 5 points per level of magical aptitude. Zun and Glaethryn, however, can buy humours which are equivalent to unlimited Magical Aptitude, and which cost the same. The Wexivanov are a special case; male Wexivanov can learn spells from about half the colleges, and female Wexivanov can learn spells from the other half.
Psi follows the GURPS psionics rules, but, somewhat as with magical aptitude, psi is only available to a few races (the Jhray, Jhotha, Wexivinov, Briacite), most of whom have counterbalancing disadvantages. These only have telepathy and telekinesis powers. (However, the specialized humours of some other races are basically equivalent to a psi power limited to a certain skill, e.g. the Slithu illusionary humours and the Llegisia Dequalant humour.)
Most regions of the Caligoi, through most of history between the Sundering and the Rejoining, are at tech level 2 or 3, except with regard to medicine, in which case magic and humours make the tech level effectively 6 (as though healing mages had access to anaesthetics and simple antibiotics, but not to complex prostheses). In many places the Pliv are at tech level 4 while their above-ground neighbors are still at tech 3.
The Beautiful and Ugly advantages and disadvantages only affect reaction rolls for characters of the same race (or a closely related race such as Slithu & Briacite), so GMs may allow them at 2/3 standard cost in some campaigns.