You've dealt with a lot of deaths before, Sebastian. So you'll be aware of when I'm about to die. [It's not an accusation or judgment in the least, just a statement of fact delivered as he touches his fingers to Sebastian's temple.] Promise me you'll finish me off. I want my last heartbeat to remain with you.
[Call it a last parting gift, one that Ciel hopes will haunt Sebastian until the day he ceases to exist.]