FYI: Twinkle has a unique cosmetic drop for imp - you will be locked out after questline
Make sure to kill him before progressing too far, otherwise no known way to go back. Hope blizz sees this and adds a way to go back, really unfair to lose a unique drop because no ones killing it.
Hello Warlocks! We have heard your questions through the Twisting Nether and would like to make some clarifications.
While you may defeat Twinkle during the new Fractures in Time Warlock questline to receive one of your new imp customization grimoires, there is another way.
If you return to the Darkmoon Faire during normal business hours, you may find something waiting for you inside the Darkmoon Deathmatch.
We hope that this helps relieve any concerns related to permanently missing one of the grimoires.
Happy summoning, and best of luck in collecting all of your various new demons!