Lurking Sellsword

When Lurking Sellsword enters the battlefield, if
you cast it from your hand, the next time a
creature enters the battlefield under the control
of an opponent, Lurking Sellsword deals 4
damage to that creature. (This triggers only if
Lurking Sellsword is on the battlefield.)
"It was a trap. Set up by those I call my brothers, to
end my life."