https://www.youtube.com/watch?v=IXmsztPvz8Y
This is Clogged Toilet by Littlemonk5. It is a classic minigame that has been in every Halo game since Halo 3 and has been remade once again for Halo 5 (with a quite impressive face lift). What happens is that the humans have to survive for 2 minutes while the zombies come up from the toilet and try to infect them. It might seem that it’s simple to survive but once a few people get infected it’s next to impossible to hold off the zombies coming from the toilet. It’s a ton a fun and you should definitely give it a try.