Glowing Light Trail - Eevee Trying to make this effect in eevee:
https://steamcommunity.com/sharedfiles/filedetails/?id=1629004850
I'm looking to make a glowing trail of light, in eevee, which starts of bright but tapers away into darkness. It needs to be able to be locked to follow the motion of another object.

[solution]:

I have a stream of particles following the eye: newtonian physics, 0 gravity, 0 velocity. They fade from bright to nothing using the process @atomicbezierslinger gave.
While the particles do follow the path as hoped for, they don't 'connect up' yet. The easy fix (in the above image) is just to have a high 'Number' (2500 for each eye in this case).
I had an issue where the particle appeared in 'clumps' every frame. This was because it was parented to an animated object which moved only on every frame. Baking the motion and removing the emitter's parent fixed the problem.
I'll try the process @RichSedman linked later. It may well be a better solution.
Thank you for the advice.
Size and other attributes can be controlled by percent of life expired with a texture associated with the particle system. Blend texture horizontal and color ramp.  Strand Particle Coordinates. Below size is decreased as particles age.
Please click on images to see them larger.

Materials can use [particle info node] to calculate percent of life expired.

Please experiment and improve the information provided.  You can do a better job.
There is also Bloom.  Highlight Blurring.  Cycles may be preferable for transparency.  There are may videos on the site with the names that rhymes with noonoob and here at BSE.  The Blender compositor can select the highlight and blur.
value = [Current age] / [lifetime] with a math node.
So a material can become transparent or change color for a particular particle over time.  I have chosen to change color for a quick answer which you can improve.
This work with practice will take less than 15 minutes for a rough approximation.
The older particles can be more transparent and smaller and less bright in the four combinations.