〜たる(もの)
anyone who is (a …) should; as one who is
Noun (role) + たるもの · Noun + たる + 者 / Noun
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たるもの sets out the duties or ideals of a role: 教師たるもの、生徒の手本でなければならない, "a teacher must be a model for their students".
たる is a classical form of である, so 教師たる者 is "one who is a teacher". The second half states what such a person must, should or must not do: なければならない, べきだ, てはいけない. The tone is lofty and moralising, like a code of conduct.
Before another noun, it's just たる: 一国のリーダーたる者, "one who leads a nation".
It's a stronger, more old-fashioned version of として (N3). Compare たるや, which only emphasises a topic.
Examples
教師たるもの、生徒の手本でなければならない。
A teacher must be a model for their students.
医者たるもの、患者の命を第一に考えるべきだ。
Anyone who is a doctor should put the patient's life first.
社会人たるもの、時間は守らなければならない。
A working adult must be punctual.
一国のリーダーたる者は、国民の声を聞くべきだ。
One who leads a nation should listen to its people.
男たるもの、人前で泣いてはいけないと言われて育った。
I was brought up being told that a man must never cry in front of others.