When do Warlocks learn Sacrifice?

Rank 1 at level 16, with later ranks at 24, 32, 40, 48, 56. Visit a class trainer in any capital.

Did Sacrifice change from Classic?

No recorded differences from Classic 1.12 in this snapshot.

Rank	Level	Cost / range / cast	Effect	Classic 1.12
Rank 1	16	Unlimited range · Instant · Demon: Voidwalker	Sacrifices the Voidwalker, giving its owner a shield that will absorb 318 damage for 30 sec. While the shield holds, spellcasting will not be interrupted by damage.	same
Rank 2	24	Unlimited range · Instant · Demon: Voidwalker	Sacrifices the Voidwalker, giving its owner a shield that will absorb 528 damage for 30 sec. While the shield holds, spellcasting will not be interrupted by damage.	same
Rank 3	32	Unlimited range · Instant · Demon: Voidwalker	Sacrifices the Voidwalker, giving its owner a shield that will absorb 793 damage for 30 sec. While the shield holds, spellcasting will not be interrupted by damage.	same
Rank 4	40	Unlimited range · Instant · Demon: Voidwalker	Sacrifices the Voidwalker, giving its owner a shield that will absorb 1,123 damage for 30 sec. While the shield holds, spellcasting will not be interrupted by damage.	same
Rank 5	48	Unlimited range · Instant · Demon: Voidwalker	Sacrifices the Voidwalker, giving its owner a shield that will absorb 1,503 damage for 30 sec. While the shield holds, spellcasting will not be interrupted by damage.	same
Rank 6	56	Unlimited range · Instant · Demon: Voidwalker	Sacrifices the Voidwalker, giving its owner a shield that will absorb 1,930 damage for 30 sec. While the shield holds, spellcasting will not be interrupted by damage.	same

Improved Voidwalker (Demonology row 2): Increases the effectiveness of your Voidwalker's Torment, Consume Shadows, Sacrifice, and Suffering spells by 30%.
Demonic Sacrifice (Demonology row 3): When activated, sacrifices your summoned Demon to enhance the opposing aspect of your power, granting you an effect that lasts 2 hrs. The effect is canceled if any Demon is summoned. Imp: Increases your Shadow damage by 15%. Voidwalker: Restores 2% of your total Mana every 4 sec. Succubus/Incubus: Increases your Fire damage by 15%. Felhunter: Restores 3% of your total Health every 4 sec.
Demonic Pact (Demonology row 7): Your Demonic Sacrifice effect is no longer cancelled by summoning a different Demon pet. Resummoning the sacrificed pet will still cancel the effect.

Source: beta client 1.60.1.70753. Spell ID 19443.