Gandhari obtained the boon for a 100 sons from Shiva
He heard from the Brahmanas that Subala's daughter, the fortunate Gandhari, had obtained a boon by pleasing Hara, the god who robbed Bhaga of his eyes. She would have 100 sons.
- 103, Sambhava parva, Mahabharata.
Amba obtained the boon to be reborn as Shikhandi from Shiva
The Mahadeva, with the bull on his banner, spoke truthfully to the maiden. ‘O fortunate one! I do not utter false words. What I have said will come true. You will attain manhood and will kill Bhishma in battle. When you enter another body, you will remember everything. You will be born as a maharatha in Drupada’s lineage. You will be an extremely honoured warrior who is swift in the use of weapons. O fortunate one! Everything will be exactly as I have said it will be. You will become a man after some time has passed.
- 188, Amba Upakhyana parva, Mahabharata.
Kalayavana had a boon of invincibility against the Yadavas from Shiva (Krishna honoured this boon and so had Kalayavana killed by Muchukunda instead)
For twelve years, he (Garga) only survived on powdered iron. He thought of Mahadeva and worshipped the wielder of the trident. Rudra granted him the boon that he would obtain a son who would possess every kind of energy and would be able to vanquish the Vrishnis and the Andhakas in battle. The supreme lord of the Yavanas heard about this boon of the birth of a son.
- 85, Harivamsa, Mahabharata.
Arjuna obtained Pashupatastra from Shiva
"I will give. O Pandava! My own weapon, roudra, will always be present before you." Satisfied, the lord granted me that pashupata weapon.
- 163, Yaksha Yuddha parva, Mahabharata.
Shiva granted the boon to Jayadratha that allowed him to block the Pandavas (except Arjun) outside the chakravyuh (Arjuna was protected by Krishna and hence was invincible)
They (Pandavas) are invincible in battle and cannot be killed in battle. But you will be able to restrain them, except for the mighty-armed Arjuna, whom even the gods find impossible to assail. He is foremost among those who are skilled in the use of weapons. He is protected by Krishna, known as the one who cannot be vanquished, and who holds the conch shell, the chakra and the mace." Having been told this, the king went to his own abode.
- 256, Draupadi Harana parva, Mahabharata.
Draupadi's boon in her previous life of having 5 husband's were also granted by Shiva
Then the god Shankara happily granted her the boon and said, ‘O fortunate one! You will have five excellent husbands.’ The one who had pleased the god said, ‘O Shankara! I wish to have only one husband who possesses all the qualities.’ The god of gods, extremely pleased with her, again uttered these holy words. ‘You have addressed me five times, asking for a husband. O fortunate one! It shall be as you have asked. You will have good fortune and all this will happen in one of your future births.’ O Drupada! So this daughter, with the form of a goddess, was born to you. “
- 189, Vaivahika Parva, Mahabharata.
Bhurishrava was stronger than Satyaki because of a boon from Shiva
The lord Mahadeva, supreme among the granters of boons, was satisfied. He wished to gratify him with a boon and the king asked for the following boon. "O lord! I ask for a son who will strike down Shini's descendant in the midst of thousands of kings and kick him with the foot in a battle." Having heard the words of King Somadatta, the god agreed that it would be this way and disappeared.
- 119, Jayadratha vadha parva, Mahabharata.
Finally, Shiva empowered Ashwatthama to destroy the remaining Pandava army
I have protected the Panchalas and exhibited many different kinds of maya. I have protected the Panchalas and have shown him honour. However, they have been overtaken by destiny and can no longer remain alive now. Having spoken these words to the great archer, the illustrious one entered his (Ashwatthama’s) body.
- 7, Sauptika parva, Mahabharata.