In Effects of being hit by an object going at FTL speeds, the effects of a spear going at FTL speeds are debated in detail.
For my setting, humanity's last hope of stalling an alien invasion comes from the creation of an FTL-capable missile launched to destroy the main alien fleet. It is based on the Alcubierre Drive (I use that because there is plenty of resources on that topic), the same type of warp drive used in the aforementioned post.
What would it look like? (since the setting I am currently developing will be mostly used in visual material)
Effects mentioned in the post include "a burst of Gravitational Cherenkov Radiation", creation of extremely hot plasma if used in an atmosphere etc.
Note: I use the
science-based because I want something plausible to build the style of my universe.
EDIT: after reading the comment, it might be better to say that the fleet orbits a planet and that the missile is going to it the planet. I forgot to add the mass of the missile, probably several tons.