The setting is vampire-ridden science fantasy post-apocalypse Mexico, though it goes in different directions than what you're probably thinking right now. A lot of wacky pastiche in the style of Japanese video games--the other aesthetic influences are Norse mythology and spaghetti Westerns.
Two hundred years after the last agent of the Empire fled east back over the sea, they returned on black barges that slid over the ocean without sails. They were different this time, bloodless rather than pale, armed with an ugly magic that blotted out the sun and raised their predecessors from forgotten graves. They were vampires, the very same princelings and governors that had been defeated in life two centuries ago, transformed during their exile, coming back to claim what they lost when they yet lived.
They did not find El Sur so easy to conquer a second time. They made it to San Miguel, the ancient City of the Sun, where they rule to this day, but they have not been able to occupy any more territory. The Queen and her armies contain them, but there is much to be done. There is a constant need for itinerant exorcists and monster hunters in the hinterlands, as the vampires send their agents there to sabotage rightful rule. The great tracts of sunless, vampire-blighted lands to the east are fertile ground for saboteurs and adventurers looking for foreign gold and alien magic.
The vampires use silent black-metal machinery to blot out the sun and fly overhead on funureal barges. They tax the blood of their subjects for their own purposes and deface the holy city of San Miguel with immense necromantic diagrams. Their servants are known as the Sanguinaires.
The Queen and her forces keep the vampire in check, using devices stolen and adapted from the Old World to call forth the sun. Their allies are the Children of the Sun, heliomancers who worship gods from before the First Invasion, and the Knights del Sol, gunslinging exorcists who destroy vampires with strange machines.
And here's a paladin oath, I guess.
Oath del Sol
Brave gunslingers from the city of San Miguel, formed to defend humanity against an ancient court of vampires. They use strange machines to circumvent the defenses of their otherwise immortal enemies.
The Knights del Sol value cunning, bravery, and determination, but what they treasure most of all is results.
1st level: earth tremor, create or destroy water
2nd level: gentle repose, dust devil
3rd level: plant growth, speak with dead
4th level: control water, conjure minor elementals
5th level: control winds, commune with nature
Gun del Sol
Starting at 3rd level, you gain proficiency with firearms. You also can designate any non-thrown ranged weapon as your Gun del Sol (even if it isn't actually a gun. It's tradition) Your Gun del Sol can fire sunlight projectiles. Doing so doesn't require ammunition, but the sunlight behaves as the projectile would (so sunlight fired from bows have the same range and trajectory as arrows, and so on) and deals no damage unless natural sunlight would. However, you can use your Divine Smite ability to empower attacks made this way.
If your Gun del Sol is lost or destroyed, you must spend 8 hours modifying another non-thrown ranged weapons with divine machinery to replace it. A Gun del Sol only works for its particular Knight.
If you take a short rest in a sunny place, you can use Divine Smite at the highest available spell level on your next attack with the Gun del Sol without expending any spell slots.
Messenger of the Sun
At 3rd level, you acquire the Find Familiar ritual. In addition to the normal familiars you can use it to summon a sprite. This familiar is fully sentient regardless of type and immune to radiant damage. You receive advantage on (Arcana), Intelligence (Religion), or Intelligence (History) check pertaining to fiends or undead when you talk it out with your familiar.
At 7th level, you learn how to make lenses. Fitting a lens into your Gun del Sol takes 10 minutes, and changes the damage it deals when you empower it with Divine Smite. When You can learn how to make two lenses from the following list:
- Ruby Lens: You learn the Control Flames cantrip. When you fit this lens into your Gun, you have resistance to cold damage and your Divine Smite deals fire damage when used to empower your Gun's attacks.
- Seaglass Lens: You learn the Shape Water cantrip. When you fit this lens into your Gun, you have resistance to fire damage and your Divine Smite deals cold damage when used to empower your Gun's attacks.
- Rare Earth Lens: You learn the Mold Earth cantrip. When you fit this lens into your Gun, you have resistance to lightning damage and your Divine Smite deals acid damage when used to empower your Gun's attacks.
- Quartz Lens: You learn the Gust cantrip. When you fit this lens into your Gun, you have resistance to acid damage and your Divine Smite deals lightning damage when used to empower your Gun's attacks.
- Lunar Lens: You learn the Friends cantrip. When you fit this lens into your Gun, you have resistance to radiant damage and your Divine smite deals psychic damage when used to empower your Gun's attacks.
Starting at 15th level, as an action you can summon the Piledriver, a divine machine designed to destroy wicked immortals. When you do so, you raise a circle of 6 pillars from the earth. The pillars are of Medium size and have 75 HP and 14 AC. The circle's radius cannot exceed 60 ft. Within the Piledriver, fiends and undead suffer disadvantage on all saving throws. More importantly, anyone and anything who dies in this area is dead forever, laid to rest beyond the reach of magic. Vampires, liches, death knights, and spirit nagas, and even gods are all subject to the Piledriver's effect. The effect only lasts while all Piledriver pillars stand, and ends after an hour. Your Divine Smite ability heals Piledriver pillars. You must take a long rest before you can use this ability again.
At 15th level, you gain an extra lens from the list above.
At 20th level, you can summon a minor aspect of the Sun. Once per long rest, you can cast sunburst without a material component. For an hour afterwards, a miniature sun hangs directly overhead, shedding sunlight as bright as if it were noon in a 1 mile radius around the epicenter of the sunburst. This works indoors and underground, but the miniature Sun will be outside.
At 20th level, you gain an extra lens from the list above.