I was fighting against Nere and Duergars. I almost tasted victory, but then Gale decided to die. His post-mortal animation kicked in. Nere and all the other enemies got massed healed and stopped being hostile. Nere even returned to his trap (everything got reset):

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I resurrected Gale and as thank you he became hostile, forcing me to fight him:

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The second attempt went a lot better, but this time I was really weary of Gale and made sure he didn't die.