Brutal Murder (3.5e Maneuver)

''You dramatically raise your scythe above your cowering foe. Rooted to the spot in their panic, they simply crouch there as you slice their head off.''

Make a melee attack against the target. You automatically roll a natural 20 on the attack roll, but must roll normally to confirm the critical hit. In addition, the target must make a Fortitude save (DC 15 + Dexterity modifier) or die.

Only the Fortitude save to avoid death is a Death effect; immunity to Death effects does not protect against the automatic hit.

Note that Brutal Murder is a 1-round action, not a full-round action: the target has an entire turn to get out of reach and make your maneuver effectively wasted.