Brahmacharis Can Bless And Curse

Observing brahmacharya is a tapas.
Those who observe brahmacharya develop power - power to bless and curse.
Story of Kacha and Devayani from Mahabharta is an example of this.

Devas and Asuras had frequent fights between them.
Asura Guru was Shukracharya and Devaguru was Brihaspati.
Shukracharya knew Sanjeevani vidya - he could revive the dead.
Those Asuras who got killed in the fights, Shukracharya revived them.
Devas became frustrated because Brihaspati did not know Sanjeevani vidya.

Kacha was Brihaspati's son, smart and handsome.
Devas requested Kacha to become the disciple of Shukracharya and learn Sanjeevani vidya from him.
Kacha agreed and went to Shukracharya.
Even though all was not well between Shukracharya and Brihaspati, he like Kacha's humility and behaviour and agreed to take him in as disciple.

Kacha stayed in his ashrama for 5 years.
Observed brahmacharya and learned shastras from Shukracharya.
Devayani was Shukracharya's daughter.
Very beautiful, a teenager.
She used to help Kacha but never interfering with his vow of brahmacharya.

In the meantime, Asuras came to know that Brihaspati's son was learning under Shukracharya.
They guessed, it must be for Sanjeevani vidya.
Once when he was grazing his guru's cows in the forest, Asuras killed Kacha.
Shukracharya revived him.
They killed him once again and Shukracharya again revived him.

Shukracharya didn't like the way Asuras were behaving.
He taught Kacha the Sanjeevani vidya.
Kacha had completed his studies, ended his brahmacharya vrata and was about leave the ashrama.
That time Devayani proposed, to marry him.

Kacha refused, he said - 'you are my guru's daughter, you are like my sister.'
Devayani said - 'No there is no such relationship. You are just my father's guru's another disciple's grandson, you are not my brother.'
Kacha did not budge.
Devayani cursed him - 'the Sanjeevani vidya my father taught you, it will not work for you.'
Kacha said - 'no problem, I will teach it to someone else and make it work.'

Kacha also said - 'I will not be curse you in return but you will find that you won't get a Rishi's son as your husband.'

The story ends there.
Why is this story part of Mahabharta.
Because Devayani later married Yayati, a predecessor of Pandavas and Kauravas.