From Australia, this guy has a youtube channel in which he reacts to RuneScape games, and anyone can send him videos reacting to the game. He gathers content from other streamers, so the merchandise is a way of uniting the audience by wearing his logo and the glowing purple chest. He's got 82.2K followers on youtube and is a partner of Twitch.
About the Purple Chest
He chose the chest as an element for the prints because it's a very special chest in which you open and loot it, after defeating a badass character - aparently a vampire. If a unique reward is obtained, a purple aura will emit from the chest. If no unique reward is found, players obtain up to three different common rewards instead depending on their performance. The runes spell his user name, Behemeth, and the name of the game is RuneScape ;)
RuneScape takes place in the world of Gielinor, a medieval fantasy realm divided into different kingdoms, regions, and cities. Players can travel throughout Gielinor via a number of methods including on foot, magical spells, or charter ships. Each region offers different types of monsters, resources, and quests to challenge players.
Players are represented in the game with customisable avatars. RuneScape does not follow a linear storyline; rather, players set their own goals and objectives. Players can choose to fight non-player character (NPC) monsters, complete quests, or increase their experience in the available skills. Players interact with each other through trading, chatting, or by participating in mini-games and activities, some of which are competitive or combative in nature, while others require cooperative or collaborative play.