Here’s a bit of info on how this works:
Once Scalecommander Sarkareth has been defeated for the first time on Mythic difficulty, the Cracked Titan Gem item that begins a Legendary weapon questline for Evokers will have a chance to drop from Scalecommander Sarkareth on all difficulties after the weekly reset in the region that he’s been defeated in. The item will be very rare on Raid Finder difficulty (when available) and slightly increases in drop rate with each difficulty.