Blizzard has announced that only your highest kill of Sarkareth will give you a chance at the Legendary Evoker Weapon, but it’s very unclear exactly how this works. It’s unclear exactly what this Blue Post means and I think further clarification is needed:
If you defeat Heroic first, does this mean that you’re locked out of a Mythic chance? Should people killing Mythic Sarkareth not kill Heroic, Normal or LFR before hand? How does it calculate your drop chance on higher difficulties after your current one? What are the drop chances? Quick update on some discussions we’ve seen -Defeating Scalecommander Sarkareth on the highest difficulty will cover your best chance for getting the Cracked Titan Gem that week. Read more on wowhead.com