HgI2-I + 4 HNVO3 + 2 HCl HgCl2 + 2 HIVO3 + 4 NIIO + 2 H2O

This is an oxidation-reduction (redox) reaction:

2 I-I - 12 e- 2 IV (oxidation)

4 NV + 12 e- 4 NII (reduction)

Change the oxidation states