"Legless Spider" is a side quest available that is the most labour intensive and largest payment in all of Dying Light.
When you enter Zone 0 you will get the Quest marker on your map. "Find our whos's causing the Smoke" you find Spider on the floor. He got shot and asks you to get his stuff to David but get some Hookah-Parts first.
Once you got these you meet David he will tell you to get some Meteorite Sampels these are nearby at the Lake.
Turns out David believes that the zombie virus was brought to Earth by a race of dangerous Aliens bent on the conquest of our world. As a reward for bringing him the meteorite samples David gives you a unique weapon he has developed specifically to defeat this scourge from beyond the stars...