This is a continuation of my previous mod - Leda's Sword - int-faith weapon, in its idea anyway. This mod does more and provides way more customization. More importantly it makes Rellana's swords much better. If you want to read more of my ramblings about it, you're welcome to look at the linked mod description. To the point now.

Summary
The mod adds 12 new weapon variants for Rellana's Twin Blades. For a total of 13 variants if you count the original. Each new weapon is implemented as one of the infusions, so if you infuse the weapon with, say, Magic, you get the variant for magic infusion. The base game infusion slots are listed in [square brackets]. All properties are listed for the max reinforcement level (+10).

There are two new basic weapons in the mod - Black Steel and Moon swords. Black Steel is the left hand Rellana's weapon, Moon is the right hand weapon. Most of the infusion variants below are for Moon sword, there are only two for Black Steel. I'm somewhat biased and find this Black Steel quite ugly compared to the Moon sword, but it doesn't mean it didn't get any love! Making this weapon usable took me probably 1/3 of the entire mod development time. The general idea is that Black Steel only gets the option to use ashes of war, but otherwise stays the same weapon while the Moon sword gets a lot of versatility.

[Standard] Rellana's Twin Blades as it is in the vanilla game. No changes were made except for allowing to change ash of war, no way around this change as it's required to get access to other variants. As a bonus you get to use various ashes of war with the vanilla weapon variant.
Hit VFX: Carian magic/Messmer flame
Scaling: C/C/C/C
Requirements: 13/16/16/16

[Heavy] Black Steel Straight Sword. The left hand Rellana's weapon wielded as a straight sword. Standard straight sword moveset, no special properties. Fire damage.
Hit VFX: Messmer flame
Scaling: D/D/-/B
Requirements: 15/13/-/17

[Keen] Black Steel Light Greatsword. The left hand Rellana's weapon wielded as a light greatsword. Standard light greatsword moveset, no special properties (*1). Fire damage.
Hit VFX: Messmer flame
Scaling: D/D/-/B
Requirements: 13/15/-/17

[Quality] Onyx Moon. Gravity infusion. This infusion provides slight flat magic base damage that doesn't scale and scales physical damage with Int. Raises physical (+40) and magical (+70) attack power after a series of successive hits and adds gravity weapon buff VFX for 7 seconds. +20% damage to gravity type enemies (like Alabaster Lords) in PvE.
Hit VFX: Gravity magic
Can be buffed: Yes (only this one can be)
Scaling: E/E/B/-
Requirements: 13/13/19/-

[Fire] Flaming Moon. Simple fire damage infusion.
Hit VFX: Messmer flame
Scaling: E/E/-/A
Requirements: 13/13/-/17

[Flame Art] Burning Moon. Fire infusion like Flaming Moon, but with reduced immediate damage. Inflicts continuous fire damage for a very short time (similar to blackflame blade).
Persistent VFX (*2): Messmer flame
Hit VFX: Messmer flame
Scaling: E/E/-/B
Requirements: 13/13/-/19

[Magic] Magical Moon. Simple magic damage infusion.
Hit VFX: Carian magic
Scaling: E/E/A/-
Requirements: 13/13/17/-

[Cold] Freezing Moon. Magic infusion like Magical Moon, but with reduced immediate damage. Inflicts continuous magic damage for a very short time (similar to blackflame blade). Applies frostbite status buildup (fixed 60).
Persistent VFX (*2): Carian magic
Hit VFX: Magic/Frost
Scaling: E/E/-/B
Requirements: 13/13/19/-

[Lightning] Ardent Moon. Fire infusion like Flaming Moon, but with reduced damage. Charges weapon with lightning (+70) after a series of successive hits and overlays (*3) lightning weapon and body buff VFX for 7 seconds. While in charged state, hits with the weapon release lightning explosion dealing 275 + 5% of the target's maximum HP damage. The explosion has a cooldown of 3.5 seconds, so it can be triggered at most twice per charge with correctly timed hits. The cooldown expires when the lightning overlay on the target's body disappear. The effect on the target also reduces fire and lightning negation by 15% (10% PvP), so there's a tradeoff between combo attacking with increases damage for reduced negation or timing attacks for explosion chains.

It took me quite a while trying to think of something to fit into [Lightning] infusion slot. Other infusions came naturally, but not this one. Then I saw Bayle's Flame Lightning and decided on this variant. Unfortunately there's no flame lightning weapon buff VFX in the game, so I had to improvise.

Persistent VFX (*2): Giantsflame, lightning overlay when charged.
Hit VFX: Giantsflame, Bayle's Flame Lightning replaces it when charged.
Scaling: E/E/-/B
Requirements: 13/13/-/22


[Sacred] Radiant Moon. Int/faith infusion with magic damage. Raises magic attack power (+70) after a series of successive hits and adds Loretta's weapon buff VFX for 7 seconds.
Hit VFX: Carian magic
Scaling: E/E/A/A
Requirements: 13/13/19/19

[Poison] Withered Moon. Magic infusion, less damage than Magical Moon. Applies deadly poison status buildup (fixed 72). Has slight arcane scaling, but arcane only scales status buildup, doesn't affect damage.
Persistent VFX (*2): Poison blade
Hit VFX: Poison
Scaling: E/E/B/-/C
Requirements: 13/13/19/-/-

[Blood] Aberrant Moon. Fire infusion, less damage than Flaming Moon. Applies hemorrhage status buildup (fixed 70). Continues to build up onset of blood loss for a very short time (similar to bloodflame blade). Has slight arcane scaling, but arcane only scales status buildup, doesn't affect damage. 
Persistent VFX (*2): Bloodflame blade
Hit VFX: Bloodflame/Blood dripping
Scaling: E/E/-/B/C
Requirements: 13/13/-/19/-

[Occult] Witching Moon. Int/faith infusion with magic damage. Sorcery and incantation catalyst, scaling is close to Golden Order seal. Spell casting is performed with heavy attack input (R2), same as for Carian Sorcery Sword.
Hit VFX: Carian magic
Scaling: E/E/A/A
Requirements: 13/13/22/22

Installation

Download the file and extract its content inside ModEngine2 mods folder. Like this: "ModEngine2\Mods\Rellana's Twin Swords". The "Rellana's Twin Swords" folder should contain regulation.bin file and parts, msg and merges folders. Edit your ModEngine2 launch config to enable this mod like this:
mods = [
    { enabled = true, name = "Rellana's Twin Swords", path = "Mods\\Rellana's Twin Swords" }
]

Start the game with ModEngine2. Then you either play the DLC to get the weapon or spawn it with CE. If you're using any other mod changing the same files you'll have to merge them or only one will be loaded. Look inside the merges folder and use the provided csv files to merge into regulation.bin and json files to merge text tables.

The mod adds quite a lot of new rows to the params and while I tried not to overwrite anything in the base game, I can not guarantee any other mod you'd try to merge with this one doesn't have something with the same row id.

Notes

• Making Rellana's offhand weapon into a functional standalone light greatsword required me to copy the entire Rellana's Twin Blades moveset and tweak its damage MVs you you'd get proper damage instead of zero fire damage at times. For this reason I suspect there could be some weirdness in Black Steel LGS moveset, but my testing didn't show anything. I've done the same for the straight sword variant, but I copied the default straight sword moveset and fixed it a little for this special case. I believe the straight sword method was more clean and is likely less problematic. Again, I've found nothing while using both weapons for a while.
• Persistent VFX is, how do I put it, quite buggy. Maybe it's how the game works or how I implemented it, but the VFX has issues. Most common one is that it'd disappear if you sheathe your weapon and won't reappear when you take it back. Fixed by soft or hardswapping to the same weapon again. VFX from left hand weapon appears (and replaces) the right hand weapon VFX and vise versa. Effects applied by ashes of wars apparently don't have the same issue, so it's probably something with the way I apply my VFX. While this issue is quite jarring, I still decided to release the mod as the VFX are only visuals, the actual infusion effects are implemented differently and are applied regardless of the VFX presence. Temporary buffs have similar complication, but their limited duration makes it less of an issue.
• As I said in the infusion variant paragraph, there's no Bayle's Flame Lightning weapon buff VFX (none I found anyway). To make my improvised flame lightning VFX I simply combined two VFX together. It doesn't work all the time, notably it doesn't work properly for left hand weapons so I disabled it, for now only right hand (and two handed) stances would get the lightning overlay. To fix this properly I'd have to make my own Flame Lightning FXR, which I can, possibly, do later. Or maybe I could find another solution.
  

Mods used in the screenshots

• Rellana Carian Knight Armor (No Skirt version)
  

Tools used

• Smithbox
• WitchyBND
• FLVER Editor 2.0
• UXM Selective Unpacker