Others had asked me for a more in-depth look at how I'd used Fate Core for Crux, so this sort of helps kill two birds with one stone I hope.
There are five aspects in Crux. These replace the normal five aspects found in Fate Core. Unlike Phase Trio, each of these aspects are created after phases have been done by players, and they do not have to be tied to any specific phase.
Class Aspect: This is like High Concept, except that is must be tied to a specific Class. Classes are metaphysical in Crux- they exist, and everyone is aware of them.
Typically these take the form of [Adjective] [Class Name], but not always. They can be phrases or objects too, so long as they reflect a Class of some sort. Likewise, the selection of Class isn't limited to Pathfinder or D&D 5e or whatnot. A character could make up their own class if they wanted.
Because Classes are metaphysical and exist in even the daily lives of most people, they have their own histories. Their own societies and backgrounds.
Trouble Aspect: This remains unchanged from Fate Core.
Culture Aspect: This aspect is selected from a menu based on what culture (or species, what have you) a character is.
For each of the main races in Crux I created a menu of possible aspects players could select from. This was on purpose; I wanted to create a sense of continuity between members of the same race, but their own internal diversity. This is one of those lists of races in Crux, if you're interested in what the possibilities are.
The last two aspects are Connections. They must each connect the character to an object, another character, place or culture within Crux.
Here are the Skills for use in Crux, City of Curses. I've broken Lore down into Arcana (magical and occult knowledge), Academia (scientific and academic knowledge) and Streetwise (street know-how and criminal knowledge).
Skill List: Athletics, Brawl, Burglary, Contacts, Crafts, Deceive, Empathy, Faith, Investigate, Arcana, Academia, Streetwise, Notice, Physique, Provoke, Rapport, Reflexes, Ride, Shoot, Sorcery, Stealth, Wealth, Weaponry, Will.
There are five skill modes in Crux. Each of these modes have a racial skill tied to them. You only can access these if you have a Culture Aspect appropriate for that skill. Each skill mode typically is just five skills. However, racial skills can expand that to six.
Each mode also is based in part on the Creating NPCs part of the DMG. I combined and tweaked it into five modes, Arcane, Divine, Martial, Finesse and Skilled.
Alchemy, Arcana, Sorcery, Will, Skill Extra, Aether (Aetherblooded Skill), Seacraft (Cecaelia Skill)
Empathy, Faith, Rapport, Will, Skill Extra, Fiendblood (Tiefling Skill), Blood (Vampire/Dhampr Skill)
Athletics, Brawl, Physique, Weaponry, Skill Extra, Android (Android Skill), Skinwalking (Skinwalking Skill)
Burglary, Contacts, Deceive, Stealth, Skill Extra, Ursyklon (Ursyklon Skill)
Investigate, Notice, Reflexes, Shoot, Skill Extra, Tengu (Tengu Skill)
Skill Extras are blank tickets- characters can choose any skill as a Skill Extra and add it to the list of skills for that mode.
I also implement Approaches in addition to Skills. This complicates things a bit, but I wanted to have the intersection of "How" and Skill. I use the normal FAE approaches.
Both the Approaches and Skills have fewer points that normal. Approaches are capped at Fair, while Skills were capped at Good until I let raised them in a session a month or so ago.
I kept them capped for awhile on purpose. Using both Approaches and Skills means my Players have very high ratings when their approaches and skills align, before even invoking aspects. Keeping them capped was meant to help encourage the expansion of skills and keep players willing to spend Fate Points.
Power levels and the kinds of things their characters will be doing recently raised in difficulty and power, so raising the cap seemed appropriate.
Ala 13th Age, each character has at least 3 points they spend to represent their ties to the various Icons in Crux. When I begin writing a story arc (the situation, more or less. I try to prepare as little as I can beforehand. I'm better improvising than going off prepared notes), I roll to see what Icons are involved in the story.
An Icon's involvement means their Aspect is one of the aspects of the Story. I use this sometimes to think how or why certain characters are compelled to do things.
Characters also define their relationship with an icon, if it is positive, negative or conflicted. If I roll a 5 or 6 on a d6 at the start of story writing, that Icon is involved. A 5 means they will have some sort of cost onto the character I'm rolling for. On a 6, they are involved, but with no costs for any characters in particular.
Stunts and Extras
Stunts work like they do in Fate Core. Each I work with my players to create, as unique as possible and tailored to their character.
For Crux through, Stunts are the main means to access magical systems. Magic Items, for instance, are often stunts. Players can use these items with spending fate refresh.
If a Player spends fate refresh for a magic item, though, it means their character "owns" that item.
Ownership means I don't mess with it, or use any sort of narrative device that harms that item. Players get the final say for that sort of thing. An item could still bring them trouble, but spending the fate refresh means I will not make the item go away or be destroyed as part of some narrative tear.
If they don't "own" an item per se, I do feel like I'm within my power to mistreat an item. I tend not to do that though- I don't want to punish my players unnecessarily.
Magic is accessed through stunts.
Magic in Crux is based on flavors. When you purchase a stunt, you select two flavors of magic your character knows. Sorcery or Faith are the main skills for managing this (although one could use Alchemy or a racial skill I suppose too).
Minor effects are fiat things that have no "mechanical" effect. Things like floating in midair or creating lights. If a character wants to create an advantage or use a magic skill to cast a spell that has a mechanical effect though, they must spend 1 Fate Point, or spend 1 Spell Point.
This is meant to be a expansion on the declaration part of Fate Points. I don't get into specific spells or whatnot because I never wanted to do that much work. Nor do I want my players to have to do that much work either.
Most often we roll, a spell goes off, and we move on to the next thing as smoothly as possible.
Note that a lot of this is borne out of the Fate Core and Fate Toolkit. I tinkered around a bit to get it to fit Crux and what I wanted to do with it.
I'm always looking to tweak and alter things, especially if I think I come up with better things that'll work easier in the game.
Hope this interests you! Let me know what you think! I will expand this with other things as needed.