Remember that the Revenant Double-Bladed Scimitar weapon doesn't actually exist in the game in any way. It is only available because of a limitation with DNDBeyond where Feats cannot modify weapons - and so - in order to make the Revenant Blade feat function - they created the Revenant Double-Bladed Scimitar magic item to simulate its benefits. Without that feat - you can only ever use the Double-Bladed Scimitar and it would not be finesse.
You won't need a feat to use the Rapier unless you don't have proficiency in them - so right from the get-go the Double-Bladed scimitar has a feat tax but Rapier doesn't. Just something to take into consideration.
Okay which would do more damage as a finesse weapon?
The Revenant Double Bladed Scimitar with Great Weapon Fighting?
Or
The Rapier with Dueling?
Assuming dex 17 start and +1 to dex from the feat, the double bladed scimitar would deal 2d4+4 damage plus 1d4+4 damage for the bonus action from double bladed scimitar at 4th level with a 15 AC +1 from the feat for Leather (16) and a 16 AC +1 for Scale Mail (17). That's 9 + 6.5 = 15.5 average damage and Great Weapon fighting style will change the 2.5 average damage per 1d4 to 3 average damage pushing the 15.5 to 17. This comes at the cost of being able to do something else with the feat or having two 16s to start your character.
The rapier can start with 2 16s (say dex and con, versus the 17/15 for the double bladed scimitar) or choose something like the piercer feat for a comparable feat. The rapier deals 1d8 and either build would have an 18. That would make 1d8+4 for an average of 8.5 damage. Dueling adds +2 for 10.5. The availability of a bonus action would be dependent on the class and available feats. The dueling fighting style allows a shield to be used, so the leather armor would be 17 AC and the Scale Mail would be 18 AC.
Piercer feat would allow you to reroll one attack's damage per turn (includes OAs). The expected gain depends on what your threshold for rerolling is. If you do the 1 or 2 that Great Weapon Fighting allows, then your 4.5 per d8 would upgrade to 5.25 per 1d8. Adding in 3s would boost it to 5.44 and adding in 4s would up it to 5.5. Piercer therefore adds up to 1 average damage getting the 10.5 to 11.5. The crit bonus would push the normal 2d8 crit to a 3d8 crit, meaning that the 10.5 normal/ 15 crit upgrades to 11.5 normal/19.5 crit. Without a consistent bonus action attack, it doesn't keep up.
If you have a class that regularly uses a bonus action, then it's closer. GWFS double scimitar damage for each non bonus action attack would be 10 while the rapier would remain the 10.5/11.5 from above.
If the rapier build decided to go with a two weapon fighting style and dual wielder feat at 4, the you'd have the 16 dex still and would have the same AC as the double scimitar feat. You're attacks would be 1d8+3 for an average damage of 7.5, which means your attack plus bonus action attack at 4 would be 15. At 20 dex, that would be the same 17 average damage that the GWFS double Scimitar would have at 18 dex and 2 points behind what it would have with 20 dex. However, this does show that the rapier has more build versatility available because it doesn't have to have a particular feat to get it to work. Also note that the double scimitar would be working off of strength until the feat is taken, which means that you are either working off of a dump stat, having a fairly MAD build where the multiple attributes won't be used as much other than in the respective tiers that they would be used in, or you simply use a normal finesse weapon option until you get the feat.
Basically, they are pretty close and it depends on what you are trying to do and what visual appeals to you. The difference between 2d4 and 1d8 (or 2d6 and 1d12) is that the 2ds have a higher floor and average damage. The downside is that they don't get as much love with bonus damage to crits that come from features like piercer's crit ability, half-orc's Savage Attacks, and barbarian's Brutal Criticals. The normal crit doubles from 2dx to 4dx but each of those abilities only adds an additional dx. So a half-orc that crits would only get 5d4 from the double bladed scimitar (average of 12.5) whereas they'd get the 3d8 from a rapier (13.5).
We've already seen that the double bladed scimitar has a bonus action attack built in, which improves your round to round versatility since you don't have to change out equipment to go from a defensive stance to an offensive stance. With the feat to make it a finesse weapon, it works well with triggering sneak attack, since you can use the bonus action attack to trigger sneak attack if you miss with the main action. It gives a built in bonus attack to stack static damage such as rage damage or to get extra attacks in with Hex, Hunter's Mark, or Divine Favor. Finally, it has the bonus of only requiring one magical weapon to upgrade as opposed to the two that would be required for two weapon fighting. Many of the two weapon fighting eligible weapons are common in most adventures, so that does limit that benefit some outside of specialized builds like those that would focus on the Piercer, Slasher, or Crusher feats.
The advantage of a Double Bladed Scimitar + Revenant Blade feat, vs. two Rapiers + Dual Wielder feat:
No need for Two-Weapon Fighting Style to add your attribute modifier to the Bonus Action attack, you essentially get that style for free.
One can hold a two-handed weapon in one hand while not attacking with it, without needing to draw/stow their offhand weapon. That makes Double Bladed Scimitars better for a character who sometimes must cast spells with somatic or material components.
The Double Bladed Scimitar can benefit from the Great Weapon Fighting style (reroll 1's and 2's), which is worth a couple of average damage per round. Dual Wielded Rapier's can't benefit from Dueling to match.
Disadvantages of the Double Bladed Scimitar:
Has the "Special" property, which conflicts with some class features (e.g. Hexblade, Kensei)
Is "Two-Handed", which conflicts with some class features (e.g. Hexblade, Bladesinger)
If something comes up where you want to stop dual wielding (maintain a grapple, fight while holding onto a rope, etc.), there's no one-handed mode for the Double Bladed Scimitar, but you could sheathe one Rapier.
I was thinking either an elf or half-elf fighter with the elven accuracy feat. Probably go Samurai so that I can combine Fighting Spirit, Action Surge, Elven Accuracy, and Revenant Blade to give myself up to 8 x 2d4 dex based attacks with triple advantage at least twice per long rest. More if I get to take a short rest.
I was thinking either an elf or half-elf fighter with the elven accuracy feat. Probably go Samurai so that I can combine Fighting Spirit, Action Surge, Elven Accuracy, and Revenant Blade to give myself up to 8 x 2d4 dex based attacks with triple advantage at least twice per long rest. More if I get to take a short rest.
Revenant Doublebladed Scimitar feat and Elven Accuracy... might as well finish it off with Slasher feat. With regular attacks with advantage, you'd be scoring crits frequently and the debuffs with slasher work well for protecting the party from anyone that survives the onslaught. You could start with 17 dex, have your feats fully online by 8 as well as max dex. That would give you 2-4 levels with the full build in an average campaign and plenty of room to top off Constitution or explore other feats if going to 20.
You'll have a good usage of your bonus attack on all turns, the double scimitar one for easier fights, after you've run out of Fighting Spirit uses, or as a default option generally. When you really need to turn it on, out comes the Fighting Spirit, often coupled with an Action Surge. It's a pretty good plan.
Slasher's a good idea. Could also grab the Fey Touched Feat to get one free use of Hunter's Mark to cast on the really big targets. That could give up to an extra 8d6 when using Action Surge. It would only take one more turn to get really set up but by the end of round 3 the BBEG would really be hurting.
Using a two-handed weapon to make 2d4 attacks with your bonus going to Fighting Spirit? Meh... I don’t see the advantage of the scimitar, if you aren’t using its bonus attack. The rapier would be less feat intensive, less hand intensive, and piercer > slasher for crit fishing builds. It’s not for Samurai.
Slasher's a good idea. Could also grab the Fey Touched Feat to get one free use of Hunter's Mark to cast on the really big targets. That could give up to an extra 8d6 on top when using Action Surge. It would only take one more turn to get really set up but by the end of round 3 the BBEG would really be hurting.
An MC into Paladin or Ranger could also net you defensive fighting style and some additional Spell slots to cast Hunter's Mark with and having it from the feat would give you more options selecting from the Ranger spell list. Paladin could even grab Divine Favor to free up bonus actions when going switching from 1 enemy to another and give a compliment to the BBEG HM option. Ranger could take Swarmkeeper to get Faerie Fire to give a few more opportunities to have triple advantage if another party member doesn't provide that for you.
Using a two-handed weapon to make 2d4 attacks with your bonus going to Fighting Spirit? Meh... I don’t see the advantage of the scimitar, if you aren’t using its bonus attack. The rapier would be less feat intensive, less hand intensive, and piercer > slasher for crit fishing builds. It’s not for Samurai.
Piercer > slasher for damaging crit Fisher builds. Slasher gives some combat utility by limiting movement of one creature per turn (including OAs, though it's admittedly stronger on an 18th level Cavalier with Vigilant Defender) and gives disadvantage on all attacks to any crit target that survives. Piercer only does better if it helps to kill the creature from a defensive standpoint and that may not be what the party needs. If the OP is planning on providing more tanking ability, the control aspect will be helpful and the threat generation from the successful attacks will be nice. Fighting Spirit is only the uses per long rest. The bonus action attack will still get plenty of use and only getting the 1d4+ dex on the bonus action will likely give a good option for a slasher debuff on many turns.
At 10th, you'd potentially gain a Fighting Spirit when you roll initiative with no uses left. That will give you one round with out the bonus action attack.
Of course, you could go with something else to round out your dex and take sentinel feat if you want more tanking ability. It's a stronger option, but leaves the debuff to the reaction attack only.
Edit: Still, rapier and with piercer and maybe shield Master might be the better angle to take. You could use Resilient (Dex) to round out your dexterity and be a nightmare to damage with dex saves. Or you could use the skill expert feat and gain an extra skill proficiency along with upgrading a skill to expertise.
An MC dip would cost me my 3rd extra attack if I ever reached level 20 and possibly my 2nd action surge if I dipped too deep.
I don't think Slasher would be worth it for a Cavalier since Hold the Line and Vigilant Defender would already reduce enemy movement speed to 0.
Rapier, Shield, Res (Dex), and Shield Master had occurred to me as far more effective defensively even if I took Defensive Dualist with the other build. That's why I was wondering how much of a difference the GWF with the Revenant Blade would make damage wise, especially if you got a critical and could apply GWF to 4d4. I imagine that in the case of a critical GWF 4d4 would fall short of the 3d8 +2 granted by Piercer and Dueling.
Another option for the Revenant blade would be to take Skilled Expert to gain Expertise in Athletics. I could then grapple and shove a creature to the ground effectively restraining it before going to town on it with my remaining attacks. This would be another way to guarantee advantage besides Fighting Spirit but would only be limited to creatures no greater than 1 size larger than me. Maybe a better strategy for a Champion instead.
Shield Master+Piercer is indeed better on a crit-fishing Samurai than Revenant Blade+Slasher is. Better AC, better dex saves, bigger crits... and when you're out of Fighting Spirit, bonus action shoves to Prone can keep your advantage rolling (must make one Attack attack first, unfortunately, but then can sandwich the shove before the rest) that you're built around.
Dex at 17, +1 from Piercer to 18, ASI +2 Dex at level 6 gets you to Dex 20. At 8, Shield Master. Elven Accuracy +1 Wisdom* (or whatever) at 12, Skill Expert +1 Constitution* (or whatever) at 14, and... I dunno at 16 and 19... Medium Armor Master? Sentinel? Tough? Fighting Initiate (Defense or Blind Fighting)? Defensive Duelist? Fey Touched or Aberrant Dragonmark? Mounted Combatant? Just more ASI? Take your pick.
An MC dip would cost me my 3rd extra attack if I ever reached level 20 and possibly my 2nd action surge if I dipped too deep.
I don't think Slasher would be worth it for a Cavalier since Hold the Line and Vigilant Defender would already reduce enemy movement speed to 0.
Rapier, Shield, Res (Dex), and Shield Master had occurred to me as far more effective defensively even if I took Defensive Dualist with the other build. That's why I was wondering how much of a difference the GWF with the Revenant Blade would make damage wise, especially if you got a critical and could apply GWF to 4d4. I imagine that in the case of a critical GWF 4d4 would fall short of the 3d8 +2 granted by Piercer and Dueling.
Another option for the Revenant blade would be to take Skilled Expert to gain Expertise in Athletics. I could then grapple and shove a creature to the ground effectively restraining it before going to town on it with my remaining attacks. This would be another way to guarantee advantage besides Fighting Spirit but would only be limited to creatures no greater than 1 size larger than me. Maybe a better strategy for a Champion instead.
All good points, I'd forgot about Cavalier getting to zero out an opponent's speed.
The thing to remember about great weapon fighting is that the average damage increase is small, but it will help your damage be much more consistent. You have a 1/16 chance of rolling a 1 back to back on a d4 and your chance of rolling a 1 or a 2 drops from 50% to 25% since you would reroll the 1st of either. You already have that consistency with the 2d4 placing a higher weighting on getting 4-6 damage and GWFS pushes that weighting higher. If you are ok with doing more consistent damage in that 5-7 range from dice plus whatever modifiers, then that's the way to go. If you are okay with the swingy d8 plus modifiers with the greater crit potential then the rapier will be better. Do keep in mind that Battlemaster has Trip Attack, Feinting Attack, and Grappling Strike as ways to get advantage and that the martial Adept feat can give you an extra superiority die. They reset on a short rest which would potentially give you more advantage opportunities than Fighting Spirit, plus whatever other options you picked up.
Right, but Piercer is (essentitally) GWFS, but also leaves you open to take Dueling FS instead. Piercer+Dueling with a Rapier is strictly better damage than Slasher+GWFS with the scimitar.
Right, but Piercer is (essentitally) GWFS, but also leaves you open to take Dueling FS instead. Piercer+Dueling with a Rapier is strictly better damage than Slasher+GWFS with the scimitar.
It's not as strong as GWFS since it only does it on one damage dice on your turn, though you can choose to reroll on a 3 or 4, if you chose to do so for smaller returns on increasing risks. It does have the crit rider and the ability boost to help. As you take more attacks and add more dice to the attack, you'll get more benefit from GWFS and you still have the bonus action attack from DBScimitar for when you aren't doing Fighting Spirit or whatever else might use the bonus action. GWFS also works on OAs where piercer only activates on a crit for those and your ability to get advantage on OAs isn't as high as on normal attacks which reduces the likelihood of a crit on an OA.
Your damage won't be that much better with the rapier than with the scimitar if you aren't making the bonus action attack, just between 1-2 average damage per hit, though it could be closer to 3 with the triple advantage dropping crits frequently, but you shouldn't be using your bonus action enough otherwise to not be pulling at least close to even with those bonus attacks.
I'll admit that I'm not the biggest fan of GWFS, typically preferring the Defensive fighting style over it. However, I'd never considered that it does give you much more consistent numbers, which can be more valuable than the higher damage but higher variability available from other styles. In the end, a rapier with the dueling fighting style gives a range of 3-10 (6.5 average plus other modifiers) with an equal chance to get anything along that range. That gives a 25% chance to roll a 2 or lower and deal no more than 4 damage, which is only 22% with a 2d4 with GWFS (with a range of 2-8, average of 5.)
Still they are close enough that either option will work well for your build, it's just a question of which build will fit your group better. If you have a tank in the party already, the scimitar option gives more support for the party in general. If the party lacks a tank, the rapier will have the better personal defenses. Still personal taste will probably be a bigger factor since having fun will be a bigger deal than the difference between the builds.
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Okay which would do more damage as a finesse weapon?
The Revenant Double Bladed Scimitar with Great Weapon Fighting?
Or
The Rapier with Dueling?
The double bladed scimitar.
I have a weird sense of humor.
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Even with the guaranteed +2 melee damage from duelist on top, the double bladed scimitar still averages higher?
Multiple attacks is usually better and you get +1 to ac, so yes.
I have a weird sense of humor.
I also make maps.(That's a link)
Yes. Tell me what Dex you're working with and I can calculate it out.
Just a character concept right now but the eventual goal would be 20 dex.
Remember that the Revenant Double-Bladed Scimitar weapon doesn't actually exist in the game in any way. It is only available because of a limitation with DNDBeyond where Feats cannot modify weapons - and so - in order to make the Revenant Blade feat function - they created the Revenant Double-Bladed Scimitar magic item to simulate its benefits. Without that feat - you can only ever use the Double-Bladed Scimitar and it would not be finesse.
You won't need a feat to use the Rapier unless you don't have proficiency in them - so right from the get-go the Double-Bladed scimitar has a feat tax but Rapier doesn't.
Just something to take into consideration.
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Assuming dex 17 start and +1 to dex from the feat, the double bladed scimitar would deal 2d4+4 damage plus 1d4+4 damage for the bonus action from double bladed scimitar at 4th level with a 15 AC +1 from the feat for Leather (16) and a 16 AC +1 for Scale Mail (17). That's 9 + 6.5 = 15.5 average damage and Great Weapon fighting style will change the 2.5 average damage per 1d4 to 3 average damage pushing the 15.5 to 17. This comes at the cost of being able to do something else with the feat or having two 16s to start your character.
The rapier can start with 2 16s (say dex and con, versus the 17/15 for the double bladed scimitar) or choose something like the piercer feat for a comparable feat. The rapier deals 1d8 and either build would have an 18. That would make 1d8+4 for an average of 8.5 damage. Dueling adds +2 for 10.5. The availability of a bonus action would be dependent on the class and available feats. The dueling fighting style allows a shield to be used, so the leather armor would be 17 AC and the Scale Mail would be 18 AC.
Piercer feat would allow you to reroll one attack's damage per turn (includes OAs). The expected gain depends on what your threshold for rerolling is. If you do the 1 or 2 that Great Weapon Fighting allows, then your 4.5 per d8 would upgrade to 5.25 per 1d8. Adding in 3s would boost it to 5.44 and adding in 4s would up it to 5.5. Piercer therefore adds up to 1 average damage getting the 10.5 to 11.5. The crit bonus would push the normal 2d8 crit to a 3d8 crit, meaning that the 10.5 normal/ 15 crit upgrades to 11.5 normal/19.5 crit. Without a consistent bonus action attack, it doesn't keep up.
If you have a class that regularly uses a bonus action, then it's closer. GWFS double scimitar damage for each non bonus action attack would be 10 while the rapier would remain the 10.5/11.5 from above.
If the rapier build decided to go with a two weapon fighting style and dual wielder feat at 4, the you'd have the 16 dex still and would have the same AC as the double scimitar feat. You're attacks would be 1d8+3 for an average damage of 7.5, which means your attack plus bonus action attack at 4 would be 15. At 20 dex, that would be the same 17 average damage that the GWFS double Scimitar would have at 18 dex and 2 points behind what it would have with 20 dex. However, this does show that the rapier has more build versatility available because it doesn't have to have a particular feat to get it to work. Also note that the double scimitar would be working off of strength until the feat is taken, which means that you are either working off of a dump stat, having a fairly MAD build where the multiple attributes won't be used as much other than in the respective tiers that they would be used in, or you simply use a normal finesse weapon option until you get the feat.
Basically, they are pretty close and it depends on what you are trying to do and what visual appeals to you. The difference between 2d4 and 1d8 (or 2d6 and 1d12) is that the 2ds have a higher floor and average damage. The downside is that they don't get as much love with bonus damage to crits that come from features like piercer's crit ability, half-orc's Savage Attacks, and barbarian's Brutal Criticals. The normal crit doubles from 2dx to 4dx but each of those abilities only adds an additional dx. So a half-orc that crits would only get 5d4 from the double bladed scimitar (average of 12.5) whereas they'd get the 3d8 from a rapier (13.5).
We've already seen that the double bladed scimitar has a bonus action attack built in, which improves your round to round versatility since you don't have to change out equipment to go from a defensive stance to an offensive stance. With the feat to make it a finesse weapon, it works well with triggering sneak attack, since you can use the bonus action attack to trigger sneak attack if you miss with the main action. It gives a built in bonus attack to stack static damage such as rage damage or to get extra attacks in with Hex, Hunter's Mark, or Divine Favor. Finally, it has the bonus of only requiring one magical weapon to upgrade as opposed to the two that would be required for two weapon fighting. Many of the two weapon fighting eligible weapons are common in most adventures, so that does limit that benefit some outside of specialized builds like those that would focus on the Piercer, Slasher, or Crusher feats.
The advantage of a Double Bladed Scimitar + Revenant Blade feat, vs. two Rapiers + Dual Wielder feat:
Disadvantages of the Double Bladed Scimitar:
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I was thinking either an elf or half-elf fighter with the elven accuracy feat. Probably go Samurai so that I can combine Fighting Spirit, Action Surge, Elven Accuracy, and Revenant Blade to give myself up to 8 x 2d4 dex based attacks with triple advantage at least twice per long rest. More if I get to take a short rest.
Revenant Doublebladed Scimitar feat and Elven Accuracy... might as well finish it off with Slasher feat. With regular attacks with advantage, you'd be scoring crits frequently and the debuffs with slasher work well for protecting the party from anyone that survives the onslaught. You could start with 17 dex, have your feats fully online by 8 as well as max dex. That would give you 2-4 levels with the full build in an average campaign and plenty of room to top off Constitution or explore other feats if going to 20.
You'll have a good usage of your bonus attack on all turns, the double scimitar one for easier fights, after you've run out of Fighting Spirit uses, or as a default option generally. When you really need to turn it on, out comes the Fighting Spirit, often coupled with an Action Surge. It's a pretty good plan.
Slasher's a good idea. Could also grab the Fey Touched Feat to get one free use of Hunter's Mark to cast on the really big targets. That could give up to an extra 8d6 when using Action Surge. It would only take one more turn to get really set up but by the end of round 3 the BBEG would really be hurting.
Using a two-handed weapon to make 2d4 attacks with your bonus going to Fighting Spirit? Meh... I don’t see the advantage of the scimitar, if you aren’t using its bonus attack. The rapier would be less feat intensive, less hand intensive, and piercer > slasher for crit fishing builds. It’s not for Samurai.
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An MC into Paladin or Ranger could also net you defensive fighting style and some additional Spell slots to cast Hunter's Mark with and having it from the feat would give you more options selecting from the Ranger spell list. Paladin could even grab Divine Favor to free up bonus actions when going switching from 1 enemy to another and give a compliment to the BBEG HM option. Ranger could take Swarmkeeper to get Faerie Fire to give a few more opportunities to have triple advantage if another party member doesn't provide that for you.
Piercer > slasher for damaging crit Fisher builds. Slasher gives some combat utility by limiting movement of one creature per turn (including OAs, though it's admittedly stronger on an 18th level Cavalier with Vigilant Defender) and gives disadvantage on all attacks to any crit target that survives. Piercer only does better if it helps to kill the creature from a defensive standpoint and that may not be what the party needs. If the OP is planning on providing more tanking ability, the control aspect will be helpful and the threat generation from the successful attacks will be nice. Fighting Spirit is only the uses per long rest. The bonus action attack will still get plenty of use and only getting the 1d4+ dex on the bonus action will likely give a good option for a slasher debuff on many turns.
At 10th, you'd potentially gain a Fighting Spirit when you roll initiative with no uses left. That will give you one round with out the bonus action attack.
Of course, you could go with something else to round out your dex and take sentinel feat if you want more tanking ability. It's a stronger option, but leaves the debuff to the reaction attack only.
Edit: Still, rapier and with piercer and maybe shield Master might be the better angle to take. You could use Resilient (Dex) to round out your dexterity and be a nightmare to damage with dex saves. Or you could use the skill expert feat and gain an extra skill proficiency along with upgrading a skill to expertise.
An MC dip would cost me my 3rd extra attack if I ever reached level 20 and possibly my 2nd action surge if I dipped too deep.
I don't think Slasher would be worth it for a Cavalier since Hold the Line and Vigilant Defender would already reduce enemy movement speed to 0.
Rapier, Shield, Res (Dex), and Shield Master had occurred to me as far more effective defensively even if I took Defensive Dualist with the other build. That's why I was wondering how much of a difference the GWF with the Revenant Blade would make damage wise, especially if you got a critical and could apply GWF to 4d4. I imagine that in the case of a critical GWF 4d4 would fall short of the 3d8 +2 granted by Piercer and Dueling.
Another option for the Revenant blade would be to take Skilled Expert to gain Expertise in Athletics. I could then grapple and shove a creature to the ground effectively restraining it before going to town on it with my remaining attacks. This would be another way to guarantee advantage besides Fighting Spirit but would only be limited to creatures no greater than 1 size larger than me. Maybe a better strategy for a Champion instead.
Shield Master+Piercer is indeed better on a crit-fishing Samurai than Revenant Blade+Slasher is. Better AC, better dex saves, bigger crits... and when you're out of Fighting Spirit, bonus action shoves to Prone can keep your advantage rolling (must make one Attack attack first, unfortunately, but then can sandwich the shove before the rest) that you're built around.
Dex at 17, +1 from Piercer to 18, ASI +2 Dex at level 6 gets you to Dex 20. At 8, Shield Master. Elven Accuracy +1 Wisdom* (or whatever) at 12, Skill Expert +1 Constitution* (or whatever) at 14, and... I dunno at 16 and 19... Medium Armor Master? Sentinel? Tough? Fighting Initiate (Defense or Blind Fighting)? Defensive Duelist? Fey Touched or Aberrant Dragonmark? Mounted Combatant? Just more ASI? Take your pick.
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All good points, I'd forgot about Cavalier getting to zero out an opponent's speed.
The thing to remember about great weapon fighting is that the average damage increase is small, but it will help your damage be much more consistent. You have a 1/16 chance of rolling a 1 back to back on a d4 and your chance of rolling a 1 or a 2 drops from 50% to 25% since you would reroll the 1st of either. You already have that consistency with the 2d4 placing a higher weighting on getting 4-6 damage and GWFS pushes that weighting higher. If you are ok with doing more consistent damage in that 5-7 range from dice plus whatever modifiers, then that's the way to go. If you are okay with the swingy d8 plus modifiers with the greater crit potential then the rapier will be better. Do keep in mind that Battlemaster has Trip Attack, Feinting Attack, and Grappling Strike as ways to get advantage and that the martial Adept feat can give you an extra superiority die. They reset on a short rest which would potentially give you more advantage opportunities than Fighting Spirit, plus whatever other options you picked up.
Right, but Piercer is (essentitally) GWFS, but also leaves you open to take Dueling FS instead. Piercer+Dueling with a Rapier is strictly better damage than Slasher+GWFS with the scimitar.
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It's not as strong as GWFS since it only does it on one damage dice on your turn, though you can choose to reroll on a 3 or 4, if you chose to do so for smaller returns on increasing risks. It does have the crit rider and the ability boost to help. As you take more attacks and add more dice to the attack, you'll get more benefit from GWFS and you still have the bonus action attack from DBScimitar for when you aren't doing Fighting Spirit or whatever else might use the bonus action. GWFS also works on OAs where piercer only activates on a crit for those and your ability to get advantage on OAs isn't as high as on normal attacks which reduces the likelihood of a crit on an OA.
Your damage won't be that much better with the rapier than with the scimitar if you aren't making the bonus action attack, just between 1-2 average damage per hit, though it could be closer to 3 with the triple advantage dropping crits frequently, but you shouldn't be using your bonus action enough otherwise to not be pulling at least close to even with those bonus attacks.
I'll admit that I'm not the biggest fan of GWFS, typically preferring the Defensive fighting style over it. However, I'd never considered that it does give you much more consistent numbers, which can be more valuable than the higher damage but higher variability available from other styles. In the end, a rapier with the dueling fighting style gives a range of 3-10 (6.5 average plus other modifiers) with an equal chance to get anything along that range. That gives a 25% chance to roll a 2 or lower and deal no more than 4 damage, which is only 22% with a 2d4 with GWFS (with a range of 2-8, average of 5.)
Still they are close enough that either option will work well for your build, it's just a question of which build will fit your group better. If you have a tank in the party already, the scimitar option gives more support for the party in general. If the party lacks a tank, the rapier will have the better personal defenses. Still personal taste will probably be a bigger factor since having fun will be a bigger deal than the difference between the builds.