What are there the ways to legal transfer money from offline bitcoin core wallet to my bank account, if i have more than 100 btc?
You can't transfer bitcoin to a bank account unless you live in a dursdiciton that recognises bitcoin as a legal payment, like Japan.
So you must find someone willing to exchange fiat currency with you, then you deposit that fiat currency into your bank.
Exchanges are one way, selling directly to another party is another.
Note: Exchanges often have withdrawal limits, so find them out beforehand and I would not trust them with all of that BTC at once.
I also would be extremely wary of selling it all direct without a contract, you wouldn't sell a house without a contract, this is a similar sum of money. I'd enlist a lawyer if you find a party to sell it all at once to.
The legal aspects are taxes and money laundering compliance. Exchanges should insulate you from the money laundering issue. Then taxes: in many countries, you will need to declare capital gains taxes. To be tax efficient, consult with an accountant, with >100BTC you can afford it and can't afford not to.