4. Make a monster based on your deepest fear
My wife would say that my biggest fear is clowns, but with the remake of the movie "IT" arriving in cinemas soon, this feels like it's beaten me to the punch. Clown monsters are getting a bit passé anyway. So it might be a bit more interesting to take another route.
Other big fears would be vertigo, or the existential dread of not being who I am...
Perhaps something combining the two.
The Astral Leech is a being whose presence is detected subtly at first. A person who has drawn the attention of the Leech first notices that the colour is slowly leeched out of things over the course of several days. When travelling in the wilderness it might seem to the person that they are simply wandering into a cursed region, in urban areas (or areas where the person is familiar) these early effects might be more noticeable (an advantage is gained on perception checks to notice the effect). This gradual draining of colour represents the Astral Leech attaching themselves to the person's soul, once all colour is gone, the soul lock is complete and the person becomes the "host".
At any time after this, the Leech feeds on all emotions but the host's fear. Perhaps fear has a taste they cannot abide. As this occurs, the Host feels a sense of massive panic, they paralyse and suffer a disadvantage on all actions. The character may make a Wisdom save (or similar) to break free of this effect. Such a save fractures the connection between Leech and host, and the only way to completely sever the connection is by failing the wisdom save (or similar)...unwise characters have less fear...characters considered "fearless" due to some feat or other means, gain an advantage on the Save (because the leeches attach to them more steadfastly). Any time this save is attempted, the host is fatigued (with all the expected penalties this would normally incur), another attempt may not be made until a rest has been taken.