Dealing 100% damage to a tentacle on the lagon fight before getting sent to the lower area (second phase) causes idol prophesies to not trigger. I have several that should trigger on his death at varying corruption and after killing him on normal, and empowered (304 corruption) and not getting the idols to drop I tried holding back and not phasing him. the idols finally dropped. It seems you have to deal the killing blow to lagon in his third phase to trigger prophesies.