It is a breach of contract. Because your landlord does not agree, the lease contract you signed is a non-transferable contract. If you transfer it privately, he can unilaterally terminate the contract and will cause consequences for your breach of contract. File a request for liquidated damages and compensation for the loss.

As for you engraving a seal privately, haha, you must know the penalties under the relevant regulations. Official seals cannot be engraved casually. The public security organs also have regulations in this regard. .

As for how serious it is, I can only say that it is not a crime and will not involve criminal law. It can only be said that it is illegal and involves civil breach of contract.