I'm looking for some ideas to make it more interesting than just "perform a ritual" or defeat her spirit (a banshee) in combat.
Some inspiration, apologies for grammar errors, was a little hurried.
A character skilled in Religion will know that collecting all her remains and performing a ritual will be enough to vanquish the restless spirit, however touching the remains before calming or appeasing the spirit of your good intentions generally results in angering the entity. A Friends spell could work, but failure might result in deadly consequences for the party.
As the characters approach the remains they immediately sense that the area around the elf maiden's remains is full of malevolence and the Banshee slowly rises up from the earth to meet them...a burning anger in her eyes.
Skill Challenge
Stage 1. A Calm Emotions spell or a Persuade check (DC 13) might be able to suppress some of the emotion in the area making the Banshee more amenable to the party. Of course the Persuade check has advantage if the attempt is done using the elvish tongue.
Upon success continue to 2.
If the PCs fail, it takes them a little longer, but success is eventually attained but at a cost, refer A below, and continue to 2.
Stage 2. The Banshee, more amenable, begins to recount the horrid memories of the attackers, who viciously subdued her sister at the bottom of the staircase, then chased the elf maiden up the steps to her bedroom, she managed to locked herself in, her dog barking at her attackers as they successfully to broke down the door, then kicked her dog to the side, she remembers it whimpering in pain, her attackers moving in on her, before she then slit her own throat to escape their carnal desires...
As she tells the story, she wails (lesser) from time-to-time, causing psychic damage to those within earshot, she doesn't mean any harm, it is but a natural reaction to her recounting the story.
Save DC 13 Wisdom, success take 1d6 psychic damage, continue to point 3.
Failure, take 2d6 psychic damage, continue to point 3.
Stage 3. After she recounts the story, she falls into depression and begins sobbing even harder, it is evident she will be breaking into more wailing. Insightful characters realise that the depression will soon revert to malevolence if something isn't done to cheer her up.
DC 13 Musical Instrument, Tasha's Hideous Laughter, Bardic Inspiration ...etc breaks the Banshee's slump into depression move to point 4.
Failure reverts the PCs to 1 (skip 2 on subsequent turns).
Stage 4. The Banshee laments that she would like to free herself from this emotional prison she has created for herself.
This is cue for the PCs to collect the elf maidens remains which lie scattered around the area.
Search DC 10 success leads you to 5.
Failure refer to A below and continue to 5.
Stage 5. Religion is required to perform the ritual.
However due to the length of the elf maiden's stay her memories that keep her bound to the mortal world are extremely strong...and they slowly become visible to the PCs as whisps of smoke and ether with transparent solitary chord connecting them to the Banshee. As she sees them become visible she begins to cry. Those with Insight immediately understand that these memories need to be severed to release the restless spirit. The memories repeat themselves continuously. The transparent chords cannot be harmed.
Once a PC engages with a memory, the memory becomes invisible to the other PCs and the PCs engaged becomes the only one who can sever it.
Memory 1 - The memory of her sister being beaten and attacked by shadowy figures. As a PC engages the attacks merge becoming one solitary Shadow (1hp). If the PC is defeated, his/her strength is not lost permanently, but he/she falls unconscious (0hp) with 1 level of exhaustion, but does not become a shadow.
Memory 2 - The memory of the attackers chasing her up the stairs and attempting to break down her locked bedroom door.
A PC must make a DC 10 Strength check (not athletics). Failure knocks the PC prone and is now in combat with a Shadow (as per Memory 1)
Memory 3 - The attackers charge into the room kicking the little dog aside, it hit the furniture on the side of the room and whimpers in pain.
An Animal Handling/Healing skill check DC 13 or Healing spell is required to treat her wounded pet.
Failure results in dealing with a Shadow (as per Memory 1).
Memory 4 - As the attackers charge at the elf maiden she slits her throat with an ornate elvish dagger and life bleeds out of her. As a PC engages with this memory he needs to stop her killing herself requiring a DC 12 Dexterity check.
Failure results in dealing with a Shadow (as per Memory 1). The ornate dagger may be used and is treated as a magical weapon in this encounter.
As each memory is severed, the Banshee pauses momentarily in her crying.
Severing all the memories, releases the Banshee from her tortured existence - her last action is to offer some valuable information to the PCs.
Failure to sever any one of the memories, causes the Banshee to cry harder, before anger and hate replace her sorry state once more. Continue to 1 (ignoring 2 and any memories already severed).
A - Roll 1d6
1-3. DC's in the memories increase by 2.
4-6. Shadows in the memories have an additional +1 to hit.
