It says that the trait does not work if another creature has the trait.
I was wondering if that is meant to include enemies, or just allies.
Might well be as intended! Just wondering 
This refers to your allies only.
Thanks for the clarification! 
Lingering essence does not seem to work for me, at the moment. I fought against lister, with Tenebris on my team, and he died before some of my other creatures.