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Dark GRBs constitute a significant fraction of the GRB population. In thispaper, we present the multiwavelength analysis of an intense two-episodic GRB150309A observed early on to ~114 days post-burst. Despite the strong gamma-rayemission, no optical afterglow was detected for this burst. However, wediscovered near-infrared afterglow ($K_{\rm S}$-band), ~5.2 hours post burst,with the CIRCE instrument mounted at the 10.4m GTC. We used Fermi observationsof GRB 150309A to understand the prompt emission mechanisms and jetcomposition. We performed the early optical observations using the BOOTESrobotic telescope and late-time afterglow observations using the GTC. Apotential faint host galaxy is also detected at optical wavelength using theGTC. We modelled the potential host galaxy of GRB 150309A in order to explorethe environment of the burst. The time-resolved spectral analysis of Fermi dataindicates a hybrid jet composition consisting of a matter-dominated fireballand magnetic-dominated Poynting flux. GTC observations of the afterglowrevealed that the counterpart of GRB 150309A was very red, with H-$K_{\rm S}$ >2.1 mag (95 $\%$ confidence). The red counterpart was not discovered in anybluer filters of Swift UVOT, indicative of high redshift origin. Thispossibility was discarded based on multiple arguments, such as spectralanalysis of X-ray afterglow constrain z < 4.15 and a moderate redshift valueobtained using spectral energy distribution modelling of the potential galaxy.The broadband afterglow SED implies a very dusty host galaxy with deeplyembedded GRB (suggesting $A_{\rm V}$ $\gtrsim$ 35 mag). The environment of GRB150309A demands a high extinction towards the line of sight, demanding dustobscuration is the most probable origin of optical darkness and the very redafterglow of GRB 150309A. This result makes GRB 150309A the highestextinguished GRB known to date.

Revealing characteristics of dark GRB 150309A: dust extinguished or high-z?