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.
One who is a leader should take responsibility.
リーダーたるもの、責任を取る覚悟が必要だ。
That which is a leader must have the resolve to take responsibility.
彼には教師たる資格がある。
He has the qualifications befitting a teacher.
たる is the classical attributive form placed directly before a noun (指導者たる者), while たり is the classical conclusive form now mostly surviving in the fixed phrase 一日たりとも.
From classical Japanese auxiliary verb たり (conclusive form), with たる as its attributive form. In modern Japanese, たる survives as a formal/literary attributive auxiliary.