Warlock customization

Warlocks were given expanded customization options in the barbershop starting in Patch 10.1.5 for the [Summon Imp], [Summon Voidwalker], [Summon Felhunter], [Summon Sayaad], [Summon Felguard], [Summon Infernal], [Summon Darkglare], and [Summon Demonic Tyrant] abilities, unlocked through consumable items called grimoires primarily, or given to the warlock by default.

List of Grimoires

[Summon Imp] Customization Grimoires
Name	Source
[Grimoire of the Dreadfire Imp]	Dialogue option with Vi'el in Winterspring, making a pact for part of your life.
[Grimoire of the Fiendish Imp]	Chance to drop from Legion Timewalking bosses.
[Grimoire of the Felblaze Imp]	Chance to drop from Pusillin in Dire Maul
[Grimoire of the Felfrost Imp]	Guaranteed drop from Twinkle during the first part of the questline that starts with [30-70] A Dark Summons through dialogue options questioning Madam Shadow; alternatively, it will be in the middle of Deathmatch Pavilion when the Darkmoon Faire is active in a Forgotten Grimoire in the middle of the arena.
[Grimoire of the Netherbound Imp]	Guaranteed drop from Matron Folnuna
[Grimoire of the Darkfire Imp]	Sold by Imp Mother Dyala in Dreadscar Rift.
[Grimoire of the Trickster Fel Imp]	Chance to drop from Terestian Illhoof in Karazhan
[Grimoire of the Fiendish Imp]	Chance to drop from the final bosses of Time Rifts in the Tyrhold Reservoir in Thaldraszus.
[Grimoire of the Fel Imp]	Crafted by proficiency in Legion inscription, taught by [Technique: Grimoire of the Fel Imp]
[Grimoire of the Netherbound Imp]	Chance to drop from The Burning Crusade Timewalking bosses.

[Summon Voidwalker] Customization Grimoires
Name	Source
[Grimoire of the Voidlord]	Crafted by proficiency in Legion inscription, taught by [Technique: Grimoire of the Voidlord]

[Summon Sayaad] Customization Grimoires
Name	Source
[Grimoire of the Shadow Succubus]	Crafted by proficiency in Legion inscription, taught by [Technique: Grimoire of the Shadow Succubus]
[Orb of the Fel Temptress]	Chance to drop from Hellblaze Temptress in the Cathedral of Eternal Night in the Broken Shore in the Broken Isles.
[Grimoire of the Shivarra]	Crafted by proficiency in Legion inscription, taught by [Technique: Grimoire of the Shivarra]

[Summon Felhunter] Customization Grimoires
Name	Source
[Grimoire of the Voracious Felmaw]	Chance to drop from the final bosses of Time Rifts in the Tyrhold Reservoir in Thaldraszus.
[Grimoire of the Antoran Felhunter]	Chance to drop from the Felhounds of Sargeras in Antorus, the Burning Throne in the Antoran Wastes of Argus.
[Grimoire of the Xorothian Felhunter]	Enter Antorus, the Burning Throne in any difficulty with [Zhar'doom, Greatstaff of the Devourer] equipped or transmogged, defeat Portal Keeper Hasabel in the Halls of the Boundless Reach, enter the orange portal of the room (that corresponds do "Xoroth" during the encounter) to get to the other platform, interact with the deactivated portal found there, select the dialogue option to channel power, and the grimoire will appear on the floor after the cast time ends.
[Ritual of the Voidmaw Felhunter]	Complete Return to Karazhan until the Mana Devourer boss, go to the second floor of the Guardian's Study room, head to the southeast of the room next to the southern staircase, and there will be an interactable object in the second bookshelf from the bottom of the right bookshelf that will give you the grimoire.