If someone controls a couple nodes (10 for example, shouldn't be hard to do that right?), and at some point he isolates those nodes from the other bitcoin nodes, that will lead to an isolated blockchain that only he have access to it, and if he mines on it at 1kh/s or whatever super slow hash rate, at the time of recalculating the difficulty, it'll drop down by huge amount considering there wouldn't be any new blocks found between the time he isolated the chain till the time of changing the target difficulty (maybe he waits for 2 recalculating events, or modify something to make the difficulty drop to lowest possible not sure how possible that is). After this new low difficulty, he uses high mining power to create blocks at very high speed since the difficulty has dropped (on his own blockchain), meanwhile the true blockchain will have generated blocks that aren't included in those isolated chains, but due to him having low difficulty he generated twice the amount of blocks (or maybe x1.5 for example) if he then releases those nodes and they start interacting with other nodes, will this "self generated" chain become the "true" chain? I mean since it's a longer chain wouldn't it be the "true" blockchain? and that person would have actually reversed some transactions, and also generated new blocks and took the reward for himself?

I got this theory while reading an article about selfish mining, but for selfish mining to work you'll need to have something around 25% hash rate? (can't remember the exact %)

Anyway, the idea was taken from selfish mining, but I added that attacker needs to have couple nodes under his control and he isolate them in order to massively reduce the target difficulty so he can generate blocks at ridiculous high speed to make up for the time he didn't generate blocks while isolated.
also I'm not sure if 10 nodes are enough? maybe he needs something like 1/3 of total number of nodes to make sure that his blockchain will be published to everyone?

That's all, if anyone can explain if it's possible or not or if you can provide links to articles that help me finding the answer I'll be really appreciated, hope I explained my theory in an easy way.