Like the Korek Mechete, the SICK Bomb is another Easter Egg weapon put in by one of the developers. For the Korek Machete, Korek was actually Techland's Optimization Lead, and an all-around problem fixer. According to the PRIMA Dying Light Guidebook, "It's only natural that he'd be the type to make a Machete that is just as good at solving your problems!" In THIS case, one of Dying Light's Lead Level Designers went by the nickname "Sick," and contributed with his own joke weapon, a special Orange Tier craftable Grenade.

On the roof of the Tower, nearly under the crane, there is a checker board set up on a table, which you can use. When you use it, you make a move but can't do anything else, initially. You need to leave the Tower completely, and re-enter and take the elevator back up, go back to the board, and make the next move. You'll need to keep doing this 4-5 times, and you'll eventually win the game.

Once you've "won the game" and all black pieces are gone, leave and re-enter one more time, and a blueprint will be waiting for you: The "SICK Bomb." Pick it up. You now have the SICK Bomb to use to your free will. HOWEVER, it is very hard to craft. It makes 5 at a time, and it requires a ♥♥♥♥ TON of Underwater Algae.
The description reads: "They ate too much beans. The beans! Not the beans! ARGHHHHHHHHHH my eyes!"