In Re:Zero, Subaru has the "return by death" power, but when he tries to tell anyone about it he is stopped by what I assume is "The Witch". Why does this happen? Why can't Subaru tell anyone about his power?
The witch seems to have given him his Return by Death power and stops him when he tries to reveal it. The witch's motivations for giving him the power or keeping it secret have not been revealed yet. It seems pretty clear that there's no way for Subaru to overcome the witch and reveal the power -- when he tried to overpower the witch and reveal his ability to Emilia, the witch killed Emilia.