Imam Mahdī (mgehr) will take revenge for them:

In the sermon of Qādir, Khātam al-Anbiyā’ [The Last Prophet] consoled the world oppressed that

«اَلا اِنَّهُ الْمُنتَقِمُ مَنَ الظَّالِمِین؛

“Be aware! Indeed, he [Imam Mahdi (mgehr)] is the revenger on the wrongdoers.

و: «اَلا اِنَّهُ الْمُدرِک بِکلِّ ثارٍ لِاَولِیاءِ اللَّهِ عـَزَّ وَجـَلَّ

And be aware he is the very man who will get the revenge for friends of Allah, The Owner of Greatness and Reverence.”

Biḥār al-Anwār, V.37, P.213

islam - shia