re:zero kara kasaneru isekai seikatsu
Re:IF_Kasaneru Re:_Repeating_Life_in_Another_World_from_Zero
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).