A Nadukal also known as Hero Stone is a memorial in the Hindu tradition of Tamil Nadu. It is erected to honor a group or individual who has performed a great public service. Often it is for heroism in battle but can be for any service to the community.
The image depicted at Wat Thawaorn Wararam is that of the deceased forced laborers praying to the Hindu Deity Lord Shiva to raise them to enlightenment. Their sacrifice is deemed to be worthy of his action.
The temple in the background is one of the oldest in Tamil Nadu associated with Shiva.
https://en.wikipedia.org/wiki/Brihadisvara_Temple
After a layer of shellac was applied.