house on rent

The complaint will not have any substance against the landlord if the tenent has vacated on his own.

The fact of the case is that the first tenant moved to his new house, but he did not inform you that he was vacating your house or vacated your house. One day when he returns to his rented house he finds a stanger in the house. That amounted to criminal trespass under IPC- 441 and/or 442. The aggrieved person is entitled to make a police complaint against the new tenant and the land-lord. If the action does not come under the definitions of IPC-441 and 442, the burden of proof is on the new tenant and the land lord. 

The land lord should have given notice to the first tenant and got his reply before giving the possession to the new tenant.