1x1x1 was a supposed hacker who, according to myths was banned and later came back. There is no complete, confirmed info on 1x1x1's existance. The most reliable source of information would be the fourm post by MrDoomBringer explaining it.
In this fourm post, he talks about how ROBLOX made a joke about the "evil 1x1x1 brick" when talking about the first brick size they made (2x2). Shortly after this, a user who's name MrDoomBringer stated was "long forgotten" had found an exploit to insert scripts into any place. According to MrDoomBringer, this user told him that they wanted to show him something, and MrDoomBringer joined him in Crossroads. Once MrDoomBringer joined the user, it started raining heads that said joke messages relating to the "evil 1x1x1 brick" from earlier, saying things such as "1x1x1 is coming back!" or "He's coming for you!". After this, the user told MrDoomBringer how it worked, and he reported it to developers.
After this, other script exploits were found, and people used them. Many people who have not seen the news thread about the 2x2 brick but did hear about the crossroads hacking assumed that they were associated with the "1x1x1 hacker group". Various ROBLOX YouTubers also made videos on him, but many were just stories or lies that were later debunked.
Link to his supposed account: