Dengon's Deception

On the roll of a natural 20, Dengon's Deception glows and in Fey, inscribings appear uppon the sword. Sometimes just the word 'force' or 'shunt'. Then the bearer re-rolls the d20. On 15-20, the creature being attacked must roll a DC 19 constitution saving throw. If the creature fails, they are instantly killed by the power imbued by the soul of Dengon.