Emmerdale village will be shaken by the shock death in the evening following two weddings.
ITV has also revealed that the live special - to be shot entirely on-location - will be an hour long.
Chas Dingle (Lucy Pargeter) is set to marry Dan Spencer (Liam Fox), while Declan Macey (Jason Merrells) will take Katie Sugden (Sammy Winward) down the aisle.
Meanwhile, two babies will be born in the show's special birthday week, with Gennie Walker and Debbie Dingle both expected to go into labour.
Emmerdale's series producer Stuart Blackburn announced: "In our spectacular live episode viewers will be taken on a whirlwind journey encompassing every life experience from the two cradles to the grave with love, hate and betrayal along the way.
"We want to avoid speculation as to which character dies but suffice to say it will have major, far-reaching repercussions."
Emmerdale reaches its 40th anniversary on October 16.
> Read more about Emmerdale's 40th anniversary here