If you do not want spoilers for Tensura then do not read. (Btw, this might not be true but, I think that after Rimuru turned into a True Dragon he eventually became stronger than Veldanava, but I may be wrong. Side note, I wasn't able to know what actually happens after Rimuru becomes a True Dragon so I probably will get a lot of things wrong.) A lot of things have happened since Rimuru was reincarnated. So much, that it would pretty impossible to summarize it. After becoming stronger than Veldanava and practically becoming a god, Rimuru just got bored.