Hello!
It's difficult to answer without knowing more about the loads the wall is bearing, etc.
But spontaneously, yes, you need to make a lintel to transfer the loads to the remaining surrounding wall.
This can be done in different ways. A "long stone" you mentioned should preferably be a reinforced beam or a steel beam.
Doing a lintel in a load-bearing wall correctly requires good knowledge and is not something I recommend a DIY enthusiast to do.
The lintel should be in place before making the opening, and that requires some calculations. I recommend that you make a sketch and contact a structural engineer who can calculate the loads and create a work description for the execution of the lintel.
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