Answer

If you mean the 18th part of the Adhar Papsthanakni Sajjay, it is Mithyatva Shalya — meaning wrong faith / false belief. In Jain tradition, the Aghār Papsthanakni Sajjay is a group of 18 sajjays, and the 18th one is the section on mithyatva shalya. ( gujarativishwakosh.org)

Explanation

In Jain teachings, the 18 pap-sthanaks are the main categories of sin. The last one, mithyatva shalya, means believing false principles, false god, false guru, or false dharma as true. The word shalya means a “thorn,” so this teaching says that wrong belief becomes a thorn in the soul and blocks right spiritual progress. ( jainuniversity.org)

Spiritual Understanding

The deep lesson of this sajjay is that right faith is essential in Jain Dharma. Even if a person does outer rituals, if the inner vision is wrong, liberation is hindered. So Jain tradition strongly warns us to remove mithyatva and hold on to samyaktva — right belief. ( jainuniversity.org)

Takeaway

The 18th sajjay teaches: remove the thorn of wrong belief and keep the soul steady in right faith. That is why mithyatva shalya is considered such an important topic in Jain practice. ( jainqq.org)