Fantastic Combat Houserule for the D20 Combat System in OD&D/Whitebox

In Chainmail, and OD&D if you use it as a combat system, there is a rule for two beings of sufficient might (Heroes, Wizards, Dragons, Elementals, etc.) to engage in mortal combat.  This is called the Fantasy Combat rule and how it was resolved was through rolling 2d6 against a target number based on the strength of the opponent.  However, a single successful strike meant that the defender was slain outright, instead of losing wounds or hit points.  In this post I will briefly detail a system for replicating this facet of Chainmail's combat in OD&D.  This has been done before but I've decided to also take a crack at replicating this dramatic and deadly combat system.

There's a chance for heroics...

Two beings of 4 or more hit dice may engage in Fantasy/Mortal Combat.  They must duel mano a mano and combat proceeds as normal.  However, for every hit struck the attacker rolls a number of six sided dice equal to their hit dice to calculate damage.  This dramatic battle may not be interrupted or intruded upon by any other opponents.  Those that interrupt, strike into the duel, call for guards, etc. can only deal a single die of damage and ruin the Fantasy Combat.  After this transgression, combat reverts to its regular form.     

(Optional but fun rule) From Hell's heart...:  The gods love entertainment, and the underdog.  They will grant a PC slain in this heroic combat the strength for one last strike, a chance to spite and spell doom for their opponent.

But beware the consequences!