The saying of overhead xx usually comes from the sky overhead, which refers to the slogan of the audience's party struggle.

As a great magician in Harry Potter, Merlin is the most legendary figure in the magic world, that is, the most authoritative person. Cp parties such as Deha Party express the unshakable essence of cp with "overhead merlin".

The picture below shows FGO Meilin.

Tuyuan Baidu Encyclopedia