Lord Shiva is the only God whose deity is not worshipped but is uniquely worshipped in the Linga form.

Lord Shiva is the only God whose deity is not worshipped but is uniquely worshipped in the Linga form. The Linga symbol is an attempt to give form to the formless, supreme Brahman. When a Linga is installed on a Yoni, it represents the union of Shiva and Shakti, the beginning of creation. Puranas and Epics in Hinduism narrate why Shiva is worshipped in the form of linga. To solve the dispute, Supreme Brahman appeared before Brahma and Vishnu in the form of a pillar of fire, in Jyotirlingam form, which had no end and beginning. Jyoti is fire, whose shape is the shape of lingam. Hence the real name Jyotirlingam.‪ Shiva gave his introduction to Vishnu and Brahma ~ Gyan Swaroopam Shivoham Shivoham (Brahma), Neejanand swaroopam Shivoham Shivoham (Vishnu), Prakash Swaroopam Shivoham Shivoham (Mahesh- Shiva's 3rd eye).
As per Srimad Bhagavatam & Padma Purana recounts the story of how Shiva worship came to be restricted to the linga. A group of sages were conducting a yagna on the banks of the Saraswati river, when they started to wonder which god to dedicate the ritual to. So they sent sage Bhrigu to test who out of Brahma, Vishnu, and Shiva was most worthy. When he went to Kailash to test Shiva, Nandi prevented Bhrigu from meeting Shiva as he was sitting with Parvati. Enraged by this disrespect, Bhrigu put a curse on Shiva that he could only be worshipped in the form of a linga. In Vamana Purana, there is a different incident. Shiva goes to hermitage in the forest without any clothes and women get attracted to him. The rishis get angry and curse him. N later worship of Linga form came into practise ~ Om Namah Shivaya!