re:zero kara kasaneru isekai seikatsu

Published on April fools of 2016, Tappei Nagatsuki wrote a chapter, commonly known as Kasaneru or Greed IF, where Natsuki Subaru accepts Echidna's help and forms a contract with her. The actual chapter starts some time after the fifth arc.

This tag implicates re:zero_kara_hajimeru_isekai_seikatsu (learn more).