auxiliary verb
who are; which is; in the capacity of
Formal/literary auxiliary verb attached to a noun, often in the pattern 〜たる者 (as one who is ...) or 〜たるもの (that which is ...). Used to emphasize qualifications, roles, or inherent qualities, especially in formal writing, speeches, or set phrases.
教師たる者、常に学び続けるべきだ。
As one who is a teacher, one should always continue learning.
リーダーたるもの、責任を取る覚悟が必要
That which is a leader must have the resolve to take responsibility.