It's called a beam system, you can either:

1) Look it up in search (not recommanded as there are many horrible systems out there)

or

2) Make your own (warning: you should know what you are doing when attempting this).


If you chose #2, the neccessary things you should do is:
- Modify the New() proc of the default beam and pass the mob using it into the argument as a means of assigning the killer, damage, location, etc.

- Modify Move() to make a trail if it moves successfully, and/or check how many steps it has taken before self-deleting (if you make a step limit)

- Enter a Bump() procedure dealing damage and all that juicy stuff [warning: This is a movement procedure.. why the warning? [link] <-- read the bold part]

- GhostAnime