Neo, can you update the original post with your new taboo topic? I kind of lose track of all the things you've censored and don't want to inadvertently cast animate dead.
I agree with Criptfeind about gods being possible to kill. I like some of the older pantheons for examples, like greek gods and demigods that were constantly slaughtering each other. By the time players reach a level that lets them challenge CR 20+ encounters, they're like unto gods themselves, able to create their own planes of existence with a thought, stand at the gates of the abyss and hold back the demon hordes for eternity, and so forth.
That's not to say the encounter should be a twenty by twenty foot featureless room with a god and a poorly locked treasure chest inside. A good DM should make a story out of it that's interesting for all involved. Asmodeus can only be killed at the stroke of midnight on the last night of winter by dropping a magical bowling ball on his left big toe to prevent his immediate return to life.
Personally, I think if a DM is only having fun at the expense of his players, he's not really doing it right. I hate power-trip DMs who get their jollies by giving players impossible encounters and expect them to roll over and beg for mercy, debase themselves or otherwise feed the DMs ego. It's the same concept that makes DM NPC Mary-Sue characters so annoying. At the risk of running afoul of the alignment censorship, a DM should be True Neutral, not Lawful Evil.
That's not to say you can't have a good time at their expense. For example, I just had one of my players retire their Cleric in my Pathfinder game after 53 weeks of play. We've got a new player at the table now, running an Aasimar Cleric of Nethys, the god of knowledge and magic. She also has a background as a librarian. So in order to get her into the game, she got a quest hook from her patron and local lord to go seek out a set of very rare tomes for his private collection.
I'm grinning like a Cheshire Cat as I reveal this quest hook, and every other player groans and starts shaking their heads. This is because these particular tomes are being hunted by an NPC named Tylsa Kroft, a scholar and tomb raider who's a wanted criminal for violating crown law and trespassing into restricted areas in her quest for these tomes. About six months ago, the group encountered this NPC and rescued her on the road. She convinced the group to help her in finding the first of these tomes from a necromancer's tower right up until the group smelled a rat and realised it would violate the law to enter this area. They dumped her and thought that was the end of it.
I far prefer these sorts of problems for players to face. The NPC herself is easily within the CR of the group. Frankly, the Cleric could likely kill her with one hit. But it's the messy problem of whether to aid this known criminal that is the real challenge, and the fact it evokes such a strong response from the players even six months later means I've done my job well as a DM. It's the little touches that make the NPC so memorable, like the fact that every time they try to use Sending to get a response, she always wastes the 25 word limit on babble before answering their actual question.