The shadow claw is the hench that was given as a reward at the Boost Marathon event.

If you haven’t got this hench yet, keep getting the "Dedication Sign" with future events and quests until you get 15. You will have another chance to get it soon.

For those who already got the hench, you can formulate an even more powerful hench: 

Neo ShadowClaw level 199

Shadow Claw (3rd mix item) + NeoWindGirl (3rd mix item)