According to what I have studied yet the 23rd incarnation of Lord Shiva is (ईशान) Ishan (The ruler). The story related to this incarnation is not described much deeper or it can be my fault that I have not found a good reference yet. Can anybody help me with brief story to clear my doubts.
The story of Shiva's incarnation Ishana is told in this excerpt from the Shatarudra Samhita of the Shiva Purana. (You can read the Shiva Purana using the links in my answer here.) In this account, Brahma prays to Shiva in the Vishwarupa Kalpa, and as a result Saraswati manifests herself as his daughter Vishwarupa and Shuva manifests himself as his son Ishana. Ishana instructs Brahma, and then has four sons of his own:
As Brahma, desirous of sons, meditated on Shiva mentally, Saraswati of great sound manifested herself. She was Vishwarupa (Universe-formed). Ishana of such renowned features, supreme lord of crystal-like pure brilliance and bedecked in all ornaments manifested himself. On seeing Ishana the unborn lord, all pervading, all-bestowing, the all-in-all, of good forms as well as well as form, Brahma bowed to him. Lord Ishana after instructing the path of the good to Brahma, created four auspicious sons in collaboration with his Energy. They were born Jatin, Mundin, Sikhandin, and Ardhamunda. By means of Yoga they imparted good virtue and attained the goal of Yoga.
I assume "goal of yoga" means Kaivalya liberation, which I discuss here.
The story is also told in this excerpt from the Linga Purana. (You can read the rest if the Linga Purana here.) This account provides more detail about the prayer that Brahma says as well as what Ishana teaches Brahma (it's about the fundamental nature of Saraswati). It also provides the detail that Saraswati's incarnation Vishwarupa is a cow.