D&D 3.5 doesn’t have the same rule for either fiends or celestials. However as per the rules for conjuration magic summoned creatures can’t be killed: A summoned creature also goes away if it is killed or if its hit points drop to 0 or lower. It is not really dead.
Celestials have shown that they can be defeated, in both comics and film, but it isn’t easy . This is teased in the first Guardians movie when they take refuge in Knowhere, the head of a dead Celestial. Its cause of death is unknown, but something clearly contributed to its demise by ripping off its head.
Celestials can destroy just as easily as they create: The Second Host resulted in sinking Atlantis and the Third Host fought the Sky Gods over rights to mess with humans. Celestial Hosts can deem a planet unworthy and have it destroyed.
Is it easier to kill a celestial or a fiend?
Weird as it may seem, it is actually easier to kill a Celestial than it is to kill a Fiend. The nature of Astral Projection is that your Physical Body and your Astral Body are separated. Your Physical Body is left behind on your ‘Home’ Plane while your spirit goes walkabout.
Could anyone stand against the Celestials?
Maybe Thanos and Dr. Doom have egos big enough to counter the natural fear they inspire within people, but the Celestials are engines of impossible power. Whether serving as caretakers to the universe or culling worlds to further their mysterious cosmic motives, it’s hard not to wonder if anyone could stand against them.
What is it like being a celestial?
The Celestials are thousands of feet tall, encased in space armor, and are some of the oldest, most powerful entities in existence. They control the fate of whole galaxies through genetic engineering as well as pure destruction. It’s rare that they can look at another being and see someone that comes close to their awesomeness.
How did the Celestials come to be?
The First Celestial Host came to Earth about one million years ago, resulting in the creation of the Eternals and Deviants, as well as what would later become the mutant gene. Celestials can destroy just as easily as they create: The Second Host resulted in sinking Atlantis and the Third Host fought the Sky Gods over rights to mess with humans.
What happens to the Avengers when it rains dead Celestials?
As the sky opens up and it starts raining dead Celestials, the Avengers do what they can to mitigate the loss of life and look up into the sky and see the arrival of the Dark Celestials and the Final Host. There’s almost no limit to what these Dark Celestials can do.