We present VLT/FORS2 spectroscopy and GROND optical/near-IR photometry of the afterglow of the bright Fermi /LAT GRB 090926A . The spectrum shows prominent Lyman- \alpha absorption with N _ { HI } = 10 ^ { 21.73 \pm 0.07 } cm ^ { -2 } and a multitude of metal lines at a common redshift of z = 2.1062 \pm 0.0004 , which we associate with the redshift of the GRB . The metallicity derived from SII is \log ( Z / Z _ { \odot } ) \approx - 1.9 , one of the lowest values ever found in a GRB Damped Lyman- \alpha ( DLA ) system . This confirms that the spread of metallicity in GRB-DLAs at z \approx 2 is at least two orders of magnitude . We argue that this spread in metallicity does not require a similar range in abundances of the GRB progenitors , since the neutral interstellar medium probed by the DLA is expected to be at a significant distance from the explosion site . The hydrogen column density derived from Swift/XRT afterglow spectrum ( assuming \log ( Z / Z _ { \odot } ) \approx - 1.9 ) is approx . \approx 100 times higher than the N _ { HI } obtained from the Lyman-alpha absorptions . This suggests either a large column density of ionized gas or a higher metallicity of the circum-burst medium compared to the gas traced by the DLA . We also discuss the afterglow light curve evolution and energetics . The absence of a clear jet-break like steeping until at least 21 days post-burst suggests a beaming corrected energy release of E _ { \gamma } > 3.5 \times 10 ^ { 52 } erg , indicating that GRB 090926A may have been one of the most energetic bursts ever detected .