Seasoned roleplayers are used to things like foul monsters inhabiting pits of slime, horrible parasites that burrow straight into one's brain and need to be killed by magic, and shambling undead creatures made of rotting bodies.

To really creep them out you need to throw in a casual reference to detached body parts in a context where they never expected it.

Transcript

[SFX]: throw {Grievous grabs Obi-Wan and tosses him}
General Grievous: Sprout wings, little grub! {as Obi-Wan flies through air}
[SFX]: thud!
General Grievous: For I shall have your greedy hands.
Obi-Wan: What? What for?
General Grievous: You do not see their inner light? Snicker-snack! They are wasted on you.
Obi-Wan: Wait... You graft other people's body parts on to yourself?
General Grievous: Dookû's heart. How it bled for his cause.
Obi-Wan: You're insane!
General Grievous: Qui-Gon's tongue.
Obi-Wan: You took Qui-Gon's tongue?
General Grievous: A finer speaker never was born. I hired a shapeshifter to cut it out.
Padmé: You desecrated my body?!
General Grievous: Later, I took her eyes.
General Grievous: Do you imagine you can best the tongue of your master?
Obi-Wan: Well, yeah.
[SFX]: pull... {Obi-Wan Force Pull's Grievous's discarded blaster}
[SFX]: POW!! {He fires}