So one of my big bads for the current campaign is a Banshee.  The party is 4 level 3 characters, two of them are really rusty at RPG's.

I'm looking at the banshee and I want to make it feel like a "boss fight" and not like a "let's roll dice and see if we can beat it".  The Vissage thing feels like a good start.  Rule summary:  when you see it, you make a Wis save.  If you pass you're good.  If you fail, you're terrified for 1 minute.  You can resave and are at DisAd if you're in LOS to said Banshee.  A save ends the effect and grants 24 immunity.

Now I dig the vissage thing but I feel like it's likely to start combat with "okay, you two are out, let's hope the other two can keep things going and pull this off."  I'm wondering if I can jazz it up with some kinds of "grades" of fear?  Like they're at disadvantage until they pass a higher DC save?  Default is 13.  

Or am I over-thinking this because the visage is an action so the Banshee won't be going Boo Boo Boo every turn if it wants to live.  It'll be doing that periodically...

I also had the thought to change it up so that at half health it drops it's spooky look and goes "Full Banshee" and that's when the visage fires off as a free action.  That would set up the second phase of the fight and put them in a position to have deal with the horrible look and then the "wail" which I'm hoping will force the party to hunker down and cover their loses.

Thoughts?

The banshee would really only use visage once, right at the start of combat. Since people who make the save are immune for 24 hours, there’s no point in doing it a second time — everyone has either made the save (and are immune) or not (and can’t be double terrified). 
Usually that leaves a character or two who made the save, while the others are continuing to try and save, and eventually (usually within a round or three) they make the save and join the fight. At that point, everyone has made the save, so visage wouldn’t work again. 

I would recommend giving the banshee some legendary actions, such as one use of her corrupting touch and the ability to teleport up to 120 feet. You could also use mythic actions from MOT, where when she drops to 0, she uses her Wail (instant recharge) and gains back half her HP. 

The standard Banshee isn't a very good boss fight because it's too random; essentially everything turns on the results of Wail. If three or more PCs succumb, the banshee almost certainly wins (unless the remaining PC can bring multiple allies up with a single action, such as a life cleric using preserve life). If zero or one PCs succumb, the PCs win easily.

So here is the re-written encounter.

PC's come across the young lovers in a small glen the same time the the banshee begins to scream around them.  Roll for save vs visage as it's now an auditory attack.  Fail = disadvantage until cleared. 

Banshee will then take form near the kids in an attempt to feed on them first.  Which honestly she'd bee-line for the elf in the party (way more life force but meh, maybe she's not into elves, needs work).  Party can fight her back forcing her to give ground, pushing her away from the kids.  Banshee is 3/4ths corporeal at this stage.

At half health the banshee releases effort to remain tied to the physical plane.  At this stage she can be disrupted (with sunlight or the forceful presentation of a holy item) and can be trapped in a soul jar.  There is a CHANCE the party will try to "turn" but I don't think the paladin will maintain the presentation (playing loose and fast with turning rules) once her other "phase 2 kicks in".

Now she lets lose with the "save or drop" wail.  At the same time there should be some level 1 commoners there to help collect the kids so at least a few will go down.  If anyone is felled by the wail, their life force is changed into a spectre that will fight for the Banshee and have half the remaining HP of the character that fell.  Also, any necrotic touch attacks from the banshee or it's spectre minons will heal (half?) damage dealt back to the banshee.  The party will either all drop, or have to turn and run.  Lots of good plans to make it clear the spectral force is feeding off the victims in the attacks.  

By the second time they find the banshee, with hope, they will a) know about the use of sunlight and b) will have the soul jar.

I realize this is a pretty big departure from the banshee RAW but I know the term will help the players conjure the right visual for it.

Quote from Rorlins >>

 If anyone is felled by the wail, their life force is changed into a spectre that will fight for the Banshee and have half the remaining HP of the character that fell. 

 The wail doesn’t kill anyone, it drops them to 0 hit points. They would then get death saves and could be revived by any sort of healing magic, or Otherwise stabilized. 

What your are suggesting is basically a save or die roll, which is very rare in this edition and would be a huge power upgrade for a banshee. 

Im not sure what the “ forceful presentation of a holy item” could be, (as opposed to the not forceful presentation?) or why a player would think to do something that’s not in any rule book. 

Quote from Rorlins >>

Now she lets lose with the "save or drop" wail.  At the same time there should be some level 1 commoners there to help collect the kids so at least a few will go down.  If anyone is felled by the wail, their life force is changed into a spectre that will fight for the Banshee and have half the remaining HP of the character that fell.

 That makes an already extremely swingy attack even more so.

I guess I should qualify this:  It's not "drop and die" it's "drop unconscious".  

The forceful presentation is too much flavor for "try to turn" in my description.  I was thinking about the "channel divinity" but that might only work against some of the now small army of spectres.

To be clear, I want the PC's to lose but I want them to feel a solid "we need to learn more" and "we can get it next time with a little better prep."  It is NOT intended to be just an "Interesting to kill monster; let's kill it" encounter.

Frightened is already disadvantage. It doesn't necessarily take someone out of the fight. They just can't move closer.

Any melee fighter who beat the Banshee in initiative will likely already be in melee range. They can still attack at disadvantage. Ranged fighters and spellcasters aren't much hindered by Frightened. If a melee fighter goes later in the initiative, most melee fighters still have some ranged option, even if it isn't their best option. Lastly, the Banshee's basic attack is melee, so anyone frightened and not already in melee range at least won't be taking damage.

The wail for me is a bigger problem than the visage. But keep in mind with monsters with 1/day abilities. They aren't going to know until the fight starts if the party is going to be the biggest danger they're going to face today. The way players don't use their top-level spells on the first battle of the day, monsters don't either. So narratively it will make sense if you use the wail only when the party seems to have the upper hand.