No instances of heroic killing blow... I can think of many cases where magic items have HKB - like effects, if you see what I mean. Vampire Frostblades, Lizardmen's Blade of Realities, Dark elf Venomsword, Wood elf Hagbane arrows (but they will never do anything >.>) all of these can cause an enemy to die on the spot, even though it's not technicaly heroic killing blow.
Having said that, Skulltaker only gets his HKB in a challenge, so it's not much use to you against a hydra. The Venomsword allows the assassin that uses it only three attacks, so it seems unlikely to wound a T6 monster (let alone have him fail his saves), and Hagbane arrows are not only less likely to wound, but they also require a T6 monster to fail a toughness test, so not too common. The sphinx may have heroic killing blow, but only for one attack, so I fail to see the point exactly.
That leaves Destroyer of Eternities and the Virtue of Heroism. All in all, I would say that we're going to have to wait for the new edition of army books before this becomes a very prevalent rule :/