  Dark GRBs constitute a significant fraction of the GRB population. In this
paper, we present the multiwavelength analysis of an intense two-episodic GRB
150309A observed early on to ~114 days post-burst. Despite the strong gamma-ray
emission, no optical afterglow was detected for this burst. However, we
discovered 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 observations
of GRB 150309A to understand the prompt emission mechanisms and jet
composition. We performed the early optical observations using the BOOTES
robotic telescope and late-time afterglow observations using the GTC. A
potential faint host galaxy is also detected at optical wavelength using the
GTC. We modelled the potential host galaxy of GRB 150309A in order to explore
the environment of the burst. The time-resolved spectral analysis of Fermi data
indicates a hybrid jet composition consisting of a matter-dominated fireball
and magnetic-dominated Poynting flux. GTC observations of the afterglow
revealed 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 any
bluer filters of Swift UVOT, indicative of high redshift origin. This
possibility was discarded based on multiple arguments, such as spectral
analysis of X-ray afterglow constrain z < 4.15 and a moderate redshift value
obtained using spectral energy distribution modelling of the potential galaxy.
The broadband afterglow SED implies a very dusty host galaxy with deeply
embedded GRB (suggesting $A_{\rm V}$ $\gtrsim$ 35 mag). The environment of GRB
150309A demands a high extinction towards the line of sight, demanding dust
obscuration is the most probable origin of optical darkness and the very red
afterglow of GRB 150309A. This result makes GRB 150309A the highest
extinguished GRB known to date.
