"An amulet radiating powerful magic."
Berengar's amulet is found on his corpse if Geralt chooses to challenge him to a duel on Black Tern Island at the end of Chapter IV, since naturally, Geralt wins. (If he did not, you have given up prematurely; reload and fight him again!) The amulet disconcerts Azar when he notices it, dangling from the witcher's neck presumably, during his "evil overlord" pre-victory speech.
Of course, Geralt can choose not to fight Berengar, but then he does not get this amulet. Instead, Berengar shows up to help him fight Azar Javed in his laboratory in the catacombs at the end of Chapter V.
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If you do spare Berengar's life, you earn more xp than by killing him. Later he does help you against Azar, but there he is not much help and is usually killed very quickly by the mage. For those hardy souls who manage to keep him alive for the duration of that battle, there is no extra dialogue nor reward for doing so.