This is a nymph of a shield bug in the Tessaratomidae family, commonly called tassarotimids (or sometimes "giant shield bugs" due to typically being 1.5 - 4.5 cm long).
According to Wikipedia:
They are mostly found in tropical Africa, Asia, and Oceania though a few species can be found in the Neotropics and Australia. There are about 240 species known
Of the subtaxa, only a few genera are even found in India:
Empysarus, Neosalica, and numerous in the subtribe Eusthenaria (e.g., Asiarcha, Aurungabada, Carpona, and Dalcantha). A key (Leston (1955)+)to this group of insects is available for purchase or institutional subscription on Wiley
Similar to a comment under the OP's post suggests, this page appears to suggest that the nymph of a Eusthenarian called Eurostus validus (shown below) appears quite similar to the OP's specimen. However, I cannot find definitive record of E. validus being in India.
Source: Vic wicked on Pinterest
the availability of useful information (keys, reputable websites, etc.) for this group of insects (especially for India and in English) are extremely limited, so I will just leave you here with the understanding that your insect is a tassarotimid nymph that is closely closely related to Eurostus validus, Eusthenes femoralis or some other member of the Eusthenaria.
+ Leston, D., 1955, April. A KEY TO THE GENERA OF ONCOMERINI STÅL (HETEROPTERA: PENTATOMIDAE, TESSARATOMINAE), WITH THE DESCRIPTION OF A NEW GENUS AND SPECIES FROM AUSTRALIA AND NEW SYNONYMY. In Proceedings of the Royal Entomological Society of London. Series B, Taxonomy (Vol. 24, No. 3‐4, pp. 62-68). Oxford, UK: Blackwell Publishing Ltd.