मूढग्राहेणात्मनो यत्पीडया क्रियते तपः। परस्योत्सादनार्थं वा तत्तामसमुदाहृतम् ॥

mūḍha-grāheṇātmano yat pīḍayā kriyate tapaḥ parasyotsādanārthaṁ vā tat tāmasam udāhṛitam

That austerity done under a deluded understanding, with self-torture, or with the object of destroying another, that is declared of darkness.

In plain words: Discipline done out of confusion, by torturing yourself, or in order to harm someone else is the darkest kind.