Keep in mind that this video could be taken as spoilers for the Monsters Association arc which started in Season 2 and will be a huge part of the upcoming third season of the One Punch Man anime.
The fan-made montage plays a 90-second video that is masterfully animated. The video is titled “Saitama Forgets to Hold Back and Nearly Destroys the Earth” and should give you enough hints about the outcome. In branch Reddit the creator said that this animation took only three months of work, which is incredibly impressive.
The Monster Association arc is very large. To give you an idea, it started in chapter 70 of the manga and only ended a couple of months ago in chapter 178. As for the anime, it started from the ninth episode of the second season. Considering that there are only three episodes left after this, this means that the arc is just getting started in the anime. At the moment, we do not know when the third season of the anime will premiere.