I think it is safe to assume that you are some kid who has just come across a bunch of spoofy bullshit on youtube and has also found interest in crypto. Internet can be crazy that way now that you don't have to go to a library or a laboratory to test what people are telling you. All you need is a video, some more videos about Tesla and other geniuses. Bet you love the Ted Talks, right?

First things first, Welcome to the forum.

Second thing; when Sci-Fi talks about an unlimited source of energy to power a super advanced civilization, it only means that the civilization has found a way to harness nearly unlimited energy. Say, from cold fusion, a Dyson sphere or in your case, unlimited energy with a throughput of multiple Gigawatts derived from a stone. (I'll not go into the details of how much continuous throughput you will actually need for a power source to match Bitcoin's hashing power). The point is that even an "unlimited source" of energy is not free. So your hypothetical 51% attacker will still have to buy all those resources.

You see, no blockchain is infallible. Its only the "cost" of an attack that determines its security. Why do you think all of those 1000 chains did not see people putting in billions of dollars? Because people with wealth are not stupid. The steps to attack a PoW chain will include raising capital, buying insane amount of miners, connecting to the network without anyone noticing (a 50% surge), paying for all that energy and then finding a time to attack.

And then, Attack what?? Crack open the 1 Million Satoshi vault?? Not possible. All you can do is a re-org and a double spend. You need to find yourself a "buyer" stupid enough to buy billions of worth of BTC and not look into the state of chain at that time.

For PoS, all you need is to buy enough of the crypto so that you are the biggest staker. That is ALL you need to attack a PoS chain. Happens all the time with smaller chains.

Now stop watching silly Youtube videos or you will end up thinking that Earth is flat. We have those loonies in our forum too. Even they don't believe that PoS is better than PoW for the foundational money of crypto.