There are four instances where Mahadeva opened his third eye.
Incident 1: Burning of Kamadeva
After reaching the place where lord Shiva was engrossed in his meditation, Kamadeva made repeated attempts to arouse passion in the heart of lord Shiva, but his actions were no avail. Right then, Kamadeva saw Parvati arriving accompanied by her companions. She was looking divine in her beauty. Just at that moment lord Shiva too had come out of his meditation. Kamadeva thought that it was the most appropriate moment to release his arrow. Kamadeva struck lord Shiva with his Kamabana which did have a deep impact on him. Lord Shiva was struck by the awesome beauty of Parvati and his heart became full of passion for her. But at the same time he was surprised at the sudden change in his behaviour. He realized that it was an act of Kamadeva. Kamadeva started remembering gods but before the deities could come at his rescue the third eye of lord Shiva got opened and Kamadeva was reduced to ashes. Parvati got scarred after seeing Lord Shiva in such a destruction anger. She went to her house along with her companions. Rati, the wife of Kamadeva wept unconsolably.
Narrative:
He saw Kama stationed on His left side with his bow fully drawn and ready to discharge the arrow. Kama was haughty and so was very senseless. O Narada, on seeing Kama in that attitude, instantaneously anger was aroused in lord Siva, the supreme soul. O sage, standing high up in the air, holding the arrow and the bow, Kama discharged his arrow, usually unerring on Siva The infallible weapon became futile on the great Lord. The furious weapon calmed down in regard to the great soul, Siva. Kama was frightened when his weapon failed. Standing there and seeing lord Siva, the conqueror of death in front, he trembled. O great sage, when his endeavor became futile, Kama who was frightened much remembered Indra and all other gods. O great sage, remembered by Kama, Indra and other gods came there, bowed to and eulogized Siva. When the gods eulogized thus, a great flame of fire sprang up from the third eye of the infuriated Siva. That fire originating, instantaneously from the eye in the middle of His forehead blazed with flames shooting up and resembling the fire of final dissolution in refulgence. After shooting up in the sky, it fell on the ground and rolled over the earth all round. Even before the gods had the time to say "Let him be forgiven, let him be excused" it reduced Kama to ashes.
Ref: Shiva Purana
Incident 2: Birth of Jalandhara
Once upon a time, Indra was going towards Kailash mountain to meet lord Shiva. Sage Brihaspati was accompanying him. Lord Shiva came to know about his arrival. He wanted to test his devotion towards him. While both Indra and Brihaspati were still on their way. Lord Shiva met them on their in the guise of a hermit. Indra did not recognize Shiva, who was sitting on the way disguised as a hermit. Indra inquired as to who he was and where he lives. Lord Shiva sat quietly without saying a word. Indra repeatedly asked the same question, but each time Shiva remained quiet. Indra became furious and tried to attack lord Shiva with his Vajra. Lord Shiva paralyzed the raised hands of Indra by his divine power. Shiva's eyes had reddened due to anger which made Indra very frightened. sage Vrihaspati was able to recognize the real identity of the hermit as to who he was. He made salutations to lord Shiva and requested him to pardon Indra. Lord Shiva became pleased and diverted the power of his third eye to the ocean. And from that energy , Jalandhara is born.
Narrative:
Indra said:"O evil-minded one, though asked you did not reply to me. Hence I am going to kill you with my thunderbolt. Who can save you?" After saying this and looking at him ferociously Indra raised his thunderbolt in order to kill him. On seeing Indra lifting up his thunderbolt, Siva prevented the fall of the thunderbolt by making his hand benumbed. Then Siva became furious. His eyes became terrible. He blazed with his burning splendor. Then Indra burnt within himself by the benumbing of his arm like a serpent whose exploits had been curbed by pronouncing magical formulas. On seeing him resplendent, Brihaspati realized immediately that he was lord Siva himself and bowed to him. Then the noble-minded Brihaspati joined his palms in reverence. He prostrated before him on the ground and began to eulogize the lord. Lord Shiva said: O Brihaspati, how can I take up the fury that has already come out of my eye? A serpent does not wear again the slough that has been cast off. Brihaspati said: O holy lord, indeed the devotees should be pitied always. O Siva, thus please make your name Bhaktavatsala (favorably disposed towards the devotees) true. O lord of gods, you deserve to cast elsewhere the fierce brilliance. 0 uplifter of all devotees, raise up Indra. Siva said: O dear one, I am delighted by your eulogy. I shall grant you the excellent boon. Henceforth you shall be famous as Enlivener because you have conferred life on Indra. I shall cast off this fire born of my eye in the forehead intended to kill Indra lest it should afflict him. On saying this he held that wonderful brilliance born of the eye in the forehead and cast it off in the briny ocean. Then the lord Rudra of great divine sports vanished from the scene. Brihaspati and Indra were relieved of their fright and they became happy. After having the immediate perception of Siva for which they had come here, Brihaspati and Indra became contented and went away to their abodes joyously.
Ref: Shiva Purana
Incident 3 : Birth of Andhaka
Goddess Parvathi as a part of sports, closed eyes of Lord Shiva from back and whole world became dark. Then lord Shiva opened His third eye and fire emanated from it and blind creature (Andhaka) was born.
Narrative:
While sporting on the eastern ridges of the Mandara mountain, Parvati sportively and playfully closed the eyes of Siva of fierce exploit. She closed the eyes with her lotus-like bands which had the lustre of coral and golden lotus. When Siva's eyes were closed, a great darkness spread immediately. By this contact with lord Siva the rapturous rutting juice exuded from her hands became hot by the fire of the eye on his forehead and Bowed out in copious drops. Conception took place and a terrible inhuman being manifested itself. It was furious, ungrateful, blind deformed, and black in color. It had matted locks of hair and fine hair all over the body. It sang, cried, laughed, danced, put out its tongue like a serpent and thundered fiercely. When this curious creature arose, Siva smilingly spoke to Parvati, Lord Siva said:- "You did it yourself by closing my eyes. O my beloved, why are you afraid of it now? On hearing these words of Siva, Parvati smilingly took off her hands from the eyes. When light spread everywhere the blind being appeared even more terrible. On seeing such a being, Parvati asked her lord Siva. Parvati said:- O lord, what is this ugly hideous being that is born in front of us. Please tell me the truth. Why was it created ? By whom ? Whose child is it ? Sanatkumara said:- On hearing these words of his beloved, the sportively inclined mother of the three worlds and the cause of creation of the blind creatures, lord Siva himself indulging in sports smilingly said;- Lord Siva said:- O Parvati of mysterious activities, listen. When my eyes were closed by you, this being of wonderfully fierce might was born of my sweat. He shall be named Andhaka.
Ref: Shiva Purana
Incident 4: A demon born from Lord Shiva’s third eye
Son of sage Dadhichi, Pippalada blamed the gods for his parents death and decided to seek revenge. Pippalada pleased Shiva through his prayers. Shiva pleased with his penance manifested and asked him whether he could see his third eye. Pipalada said he couldn't see. Shiva told him to do a penance till he could see his third eye. Pippalada therefore performed even more difficult penance for years. Eventually he managed to see Shiva’s third eye. From Shiva’s third eye, a demon was born to take revenge on the gods. Shiva persuaded Pippalada to control his anger. He convinced Pippalada that nothing can be gained by killing the gods. That would not bring his parents back. Pippalada agreed. But he wished to see his parents once. Accordingly, vimana descended from heaven on which Dadhichi and his wife were seated. They blessed Pippalada and asked him to marry and have children. However, this story doesn't involve opening of Shiva's 3rd eye.
Image (s) Courtesy: Pinterest
Footnotes