Some context here: Hellmode is an event that occurs in the depths after 5 people ring the hellmode bell. Doing this will activate hellmode and cause monster spawn rates to be increased by 10x. All monsters that spawn will spawn corrupted (Increased health and damage by 3x). To complete hellmode, you must kill 100 monsters before 20 minutes. Failure to do so, will cause the event to end abruptly, and no rewards will be granted. If hellmode is completed, a chest will spawn containing deep gems, weapons, armor, and a "diver's light plate".
More info soon (hopefully)