Not sure which mission it is. At the end there is a Hive knight and Fallen captain fighting. If you kill the knight and don't hurt the captain, the captain will turn to you, say something in Fallen speak, and leave peacefully.
English
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I don't know that I'd consider that an easter egg
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It's a alternative outcome most people wouldn't know about without looking it up or being told. Meh. Classify it however you'd like.