In an attempt to take revenge on my husband, one of my relatives seduced me into asking for divorce. After divorce, this spiteful man invented every possible lie to marry me; verbally, without witnesses or the consent of a Wali (guardian). He had sex with me several times, but when I knew that he was deceiving me and asked him for a divorce, he refused. Therefore, I started inquiring about the validity of this marriage until I got to know that it is invalid. I repented to Allaah The Almighty and asked Him for forgiveness. Now, I want to know how I should act, as he has consummated this marriage with me.
My ex-husband wants to take me back, but he does not know about all that happened.

All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.

This marriage is invalid and any sexual intercourse committed by virtue of it is nothing but adultery. Both of you -- particularly the man who fabricated lies to separate you from your ‎husband -- must repent to Allaah The Exalted and ask Him to pardon your previous misdeeds.

Abu Hurayrah reported that the Prophet, sallallaahu ‘alayhi wa sallam, said: “Whoever instigates a wife against her husband  is not from us.” [Abu Daawood and an-Nasaa’i]

However, you are entitled to receive a dowry if this man has specified a dowry for you; otherwise, you should receive a dowry that is similar in amount to dowries usually received by women of your status. The Prophet, sallallaahu ‘alayhi wa sallam, said: “Any woman who gets married without the permission of her guardian, her marriage is invalid, her marriage is invalid, her marriage is invalid. If he (i.e. the husband) had intercourse with her, then she is entitled to take a dowry in return for the intercourse he had with her. In case of dispute, the ruler is  the guardian of whoever does not have a guardian.” [Abu Daawood, Ahmad and others]

If he refuses to give you the dowry, then you have the right to file this case before a judge; otherwise, you may waive your right to take the dowry, as Allaah The Almighty Says (what means): {And if you divorce them before you have touched them and you have already specified for them an obligation, then [give] half of what you specified - unless they forego the right or the one in whose hand is the marriage contract foregoes it. And to forego it is nearer to righteousness.} [Quran 2:237]

It is permissible for you to remarry your ex-husband by virtue of a new marriage contract with all the marriage conditions fulfilled after the end of your ‘Iddah (waiting period). It is better for you to conceal what happened between you and this deceitful man from your husband knowing that you are not allowed under Sharee‘ah (Islamic law) to tell him; rather, you should make use of the concealment granted to you by Allaah The Almighty. 

Allaah Knows best.