To me is seems like the archetype is about a character that has learned to deal with negative emotions like sadness, rage and grief. The bard uses those emotions on themselves in such a way that it creates a stronger effect at the expense of affecting others. The archetype is more inward focused than other bards.
The performances probably have great personal meaning to the bard, but do not have as much meaning to someone else. For example, the performance may be about the death of his true love. To someone else this might be somewhat sad; to the bard this would be devastating. The bard has learned how to use what would normally be a crippling emotion to instead channel that emotion to something positive. Think of Romeo and Juliet, but instead of killing himself Romeo uses that grief to do incredible things.
Basically, this archetype is about a character that has experienced profound sadness and learned how to use that sadness to inspire themselves.
Cause you would think it would have something to do with the communication and manipulation of spirits ala the dirge bard. In practice tho, it seems to just be a dawnflower dervish that doesn't require you to worship saranrae.
You seem to be assuming that the author of Sorrowsoul knew about Dawnflower Dervish. I'm not convinced this assumption is correct.
The alternative is that the author really wanted to make an"emo teen Bard" archetype for kids that listen to Muse and NIN all day long. and couldn't come up with anything else mechanically that fitted the theme.*
Mechanically, you're right - Sorrowsoul is basically a nerfed but not religion-locked DFD.
*) Although there is already an entire class for them...
While some of the abilities are similar there are some differences. For one thing using Lyric of Sorrow is optional. That means the sorrowsoul can still affect other targets if he wishes. The dawnflower dervish on the other hand does not have the option to use the standard form of those performances.
The dawnflower dervish other abilities focus on using a scimitar and magic in combat. He gains DEX to damage with a scimitar and a bonus to casting defensively and being able to cast spells in combat.
The sorrowsoul is focused on survival especially vs death and undead. The saving throw bonus from darkness denied is untyped so it stacks with that of spur harm. That means a 5th level sorrowsoul can get a +6 save vs negative energy and death effects. At 11th level they can gain spell resistance as an immediate action in addition to the +2 on all saves. The DR 10 comes in fairly late so there is a good chance it may never get used on a lot of characters, but it still grants the character the ability to persevere in the face of danger.
The big difference I see is the dervish dancer is a quick agile combat focused archetype. The sorrow soul is a character that has been through a crucible of grief and emerged stronger. The dawnflower dervish will always be a DEX based combat focused character. I can see a sorrowsoul favoring CON over DEX. The sorrowsoul is a survivor that is going to be hard to put down.
using Lyric of Sorrow is optional. That means the sorrowsoul can still affect other targets if he wishes. The dawnflower dervish on the other hand does not have the option to use the standard form of those performances.
Sure... but how useful is that, really? If it makes sense for the Bard to buff the party, and the player is willing to do so, that doesn't really change throughout the campaign. Maybe if teamed up with characters that later gain competence bonuses to attack and damage? *shrugs*
The dawnflower dervish other abilities focus on using a scimitar and magic in combat. He gains DEX to damage with a scimitar and a bonus to casting defensively and being able to cast spells in combat.
Plus better action economy: Move action from the get-go, and swift action three levels earlier.
I can see a sorrowsoul favoring CON over DEX.
Why would more resistance to harsh fort-save-effects make you go for more con?
The OP was asking about the logic behind the Sorrowsoul archetype. My posts were addressing that, not which archetype is better. To me there are significant differences in the archetypes.
Being able to use the standard inspire courage and other performances allows the bard to boost the entire party including allies, summoned creatures as well as other PC’s. The usefulness of that should be pretty obvious. It gives the bard more flexibility on what he can do. You can argue which version is better but having the option to use either of them is a definite advantage. The sorrowsoul loses nothing and gains an improved ability when they need it.
The dawnflower dervish is straight up better at combat for a variety of reasons. But they give up more than the sorrowsoul. Losing Bardic Knowledge means they have to invest points in knowledge skills if they want to use them. If the dawnflower dervish puts a single point into each knowledge skill they have a decent chance of being knowing a lot of important things. Even if they don’t put a point in to a particular knowledge skill, they still get a roll on a question with a DC higher than 10.
Some death effects and most negative energy use a will save not fortitude save. There are also other things besides death effects and negative energy that use fortitude saves. Bards have good reflex and will saves so going with CON boosts his weak save. CON also give HP and determines when a character dies. If you are looking to build a bard that focuses on survival a high CON is very useful. In all honesty the sorrowsoul will probably want both but is unlikely to max out either. The dawnflower dervish on the other hand has very good reason to max out DEX. As a combat focused character with DEX as his main combat stat he is going to want his DEX as high as he can get it. Every point of bonus is a +1 to hit and damage.
The dawnflower dervish is a good archetype, but the sorrowsoul also has its advantages.
Sorrowsoul can use heavy armor
???
What the hell are you talking about?
Bards are not proficient in anything but light armor and suffer from arcane spell failure when using medium or heavier armor. Neither the dawnflower dervish or sorrowsoul changes this.
A dawnflower dervish is going to have a higher DEX, so has absolutely no reason to even attempt to use anything but light armor. A sorrowsoul could spend feats to gain proficiency in medium and even heavy armor but would still face arcane spell failure unless the spell did not have a somatic component. The sorrowsoul could spend more feats to counter this. Still spell would allow the bard to ignore the arcane failure but means using higher level spells and taking a full round. Arcane Armor Training and Master and using Mithral armor can reduce and even eliminate arcane spell failure but uses a swift action.
So, theoretically a sorrowsoul could use heavy armor, but doing so is not worth the effort.
Bards do not have proficiency with medium armor and that is a prerequisite for hellknight signifier. That means a bard needs to spend a feat for medium armor proficiency to get that in addition to taking arcane armor training before they can take a level in hellknight signifier. They also have to be able to cast 3rd level spells, which means you have to have 7 levels in bard. You have to have 2 levels of hellknight signifier to gain arcane armor mastery and even then, you still have a 15% chance of arcane spell failure, at 3rd level it is reduced to 10% and at 8th level it drops to 5%. Mithral armor can further reduce that, but you have to be at least a 5th level signifier even with Mithral armor.
Hellknight signifier does not lose any spell caster levels, but it also does not advance any bard abilities. They also have poor fortitude and reflex saves, and only 2 skill ranks per level. So, while it is possible to build a bard that can use heavy armor, they are not advancing their bardic abilities other than spell casting. To cast in armor the sorrowsoul is giving up spell resistance and DR 10/- and limiting his bardic performances to that of a 7th level bard. He does not gain the ability to use lyric of sorrow with inspire greatness or inspire heroics.
Hellknight signifier is not a good choice for a bard and even worse for a sorrowsoul. Most bard spells do have a somatic component.