The story of Tulsi in short

From a curse placed on his grandfather, King Dharmadhvaja did many penances to please Lakshmi. Lakshmi was very pleased with his devotion and incarnated as his daughter. They named Her Tulsi, for Her beauty was incomparable with any other in creation.

Soon after Her birth, however, Tulsi ran off to Badrinath and practiced severe austerities for 100,000 years with the ultimate goal of attaining Lord Narayana as Her Husband. When She completed Her austerities Brahma appeared and granted Her wish. But unbeknownst to Tulsi, Narayana had also partially incarnated as a demon king, Sankhschuda. Sankhschuda also did many years of austerity to attain Tulsi as his wife. 

Lord Brahma being pleased with him granted his desire, also. For thousands of years, TulsiTulsi was happily the wife of the demon king Sankhschuda.

Through Her one-pointed Love for Her husband and Her unshakable Faith in Her Lord Narayana, Her husband was invincible. No one could kill him. War after war he came home victorious. Of course, he began to think his power came from himself, not the ceaseless payers of TulsiTulsi.

With such pride in his mind, he attacked the Devas. They were easily overpowered.

In fear, they ran to Shiva for assistance. He agreed and sent His armies out to destroy Sankhschuda. All failed, even Muruga and Kali. Finally Shiva entered the fight, but before He was about to throw His trident, Narayana appeared to Him and asked Shiva to postpone the fight for one day.
"This evening," Narayana said, "I will enter Tulsi's chambers and take Her chastity."

As promised, Narayana appeared to Tulsi that evening, not in His true form, but in the guise of Sankhschuda. Tulsi was overjoyed to see Her husband back from thousands of years of war. In Her excitement, She was unable to see through Narayana's maya.
They spent the night enjoying themselves and in the morning when they awoke, Narayana left the bed in a very cold fashion. Her beloved husband would never do such a thing. Tulsi knew this was not Her husband.

"Who are you?! How dare you come to me in the form of my husband. Turn around and show me who you are."
Narayana turned around and showed Tulsi His true form. This was not enough to passify Her. For Tulsi knew that He came to Her so that Sankhschuda could be killed.

In Her fury She cursed Narayana, "As your heart is as cold as a stone, so I curse you to be a stone!"
Out of His endless Love for His devotee and the pain He felt for kill him. War after war he came home victorious. Of course, he began to think his power came from himself, not the ceaseless payers of Tulsi.
With such pride in his mind, he attacked the Devas. They were easily overpowered.