poisoned chalice

noun1

1. •
   Something which is initially regarded as advantageous but which is later recognized to be disadvantageous or harmful; an apparently beneficial or benign instrument or scheme for causing death or harm.

   • [W]e but teach / Bloody Instructions, which, being taught, returne / To plague th' Inuentor. This euen-handed Iustice / Commends th' Ingredience of our poyson'd Challice / To our owne lips. — 1606