A long time ago, when I was still a little kilnalan with uncontrollably steaming exhaust horns on the side of my head. The kids of the Citadel and I stumbled upon a Calamity Revenant in the dead-end of an alleyway while we were playing.
To put it simply, they were Calamity Objects that persist after the duration of the Ordeal. It was a very rare phenomenon, much rarer than the appearance of Qliphoth Object, in fact.
And thankfully, the kind of Calamity Revenant that we stumbled upon was that of a passive one.
We were only informed of the existence of Calamity Object back then, but not Calamity Revenant, so we were curious and interacted with it.
The Calamity Revenant's form was a bucket with a boiling water inside, one that couldn't be picked up, while the content of the boiling water seemed endless.