Wa `alaykum As-Salamu wa Rahmatullahi wa Barakatuh.

What you have done is called a yamin ghamus or dipping oath. It is called so because it dips a person into the hell fire. Scholars maintained that such a type of an oath does not have an expiation. Abdullah bin ‘Amr bin Al-‘as (May Allah be pleased with them) reported: The Prophet (peace be upon him) said, “(Of the) major sins are: to ascribe partners to Allah, disobey parents, murder someone, and to take a false oath (intentionally)”.

In order to gain forgiveness, you must repent to Allah by:
1. Giving up of this sin.
2. Remorse over what has been committed.
3. Having a resolve not to repeat it again in the future.

Almighty Allah knows best.