You become the Demon Lord Rimuru Tempest, gaining every skill that he owns, from common to ultimate, at the end of the series. You know every spell that he has ever learned and have a library of every item he has ever analyzed or created via great sage.
You gain access to naming— forcing a name upon a soul that was previously nameless or rewriting one that was previously there—if you are stronger than the person who gave the original name. Naming requires sacrificing some of your magic to impart the name and write it upon the soul of the one receiving the name; however, in return, the person becomes loyal to you as well as growing stronger and evolving into a stronger form than they were previously. However, if you are not a fan of forced loyalty, you can always do a form of naming called equal naming, where instead of having a master-subordinate relationship, you are now in an equal relationship.
You also have access to a pocket dimension with a replica of the great forest of Jura and the capital city, Rimuru, along with every monster he has ever named.