Alira’s Amulet is a quest item acquired after defeating Alira in the Western Forest. It is only obtainable during one of the bandit faction quests in Act 2 that requires the character to oppose Alira: “Deal with the Bandits”, “The Bandit Lord Oak”, or “The Bandit Lord Kraityn”.
If combined with Oak’s Amulet and Kraityn’s Amulet, The Apex is formed.