Does it become unlawful if a man tells his wife: "You are forbidden to me if you hide anything from me," intending to intimidate her, not to divorce her, knowing that she has indeed hidden something from him, and is their relationship considered unlawful?

The prohibition of one's wife depends on the husband's intention: if he intended divorce, it becomes a divorce; if he intended ẓihār, it becomes ẓihār. If he intended an oath or had no specific intention, an expiation for an oath (kaffārat yamīn) becomes obligatory.

Therefore, if you conditioned the prohibition of your wife on her concealing something, and she did conceal it, then the breaking of the oath has occurred. Since you intended a threat and did not intend divorce or ẓihār, an expiation for an oath is obligatory upon you.

As for the phrase "you bear the unlawful," its apparent meaning is to threaten the wife and frighten her that she will bear the sin if she conceals something, and it has no effect on the ruling.