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Overdense, metal-rich regions, shielded from stellar radiation might remainneutral throughout reionization and produce metal as well as 21 cm absorptionlines. Simultaneous absorption from metals and 21 cm can complement each otheras probes of underlying gas properties. We use Aurora, a suite of highresolution radiation-hydrodynamical simulations of galaxy formation, toinvestigate the occurrence of such "aligned" absorbers. We calculate absorptionspectra for 21 cm, OI, CII, SiII and FeII. We find velocity windows withabsorption from at least one metal and 21 cm, and classify the alignedabsorbers into two categories: 'aligned and cospatial absorbers' and 'alignedbut not cospatial absorbers'. While 'aligned and cospatial absorbers' originatefrom overdense structures and can be used to trace gas properties, 'aligned butnot cospatial absorbers' are due to peculiar velocity effects. The incidence ofabsorbers is redshift dependent, as it is dictated by the interplay betweenreionization and metal enrichment, and shows a peak at $z \approx 8$ for thealigned and cospatial absorbers. While aligned but not cospatial absorbersdisappear towards the end of reionization because of the lack of an ambient 21cm forest, aligned and cospatial absorbers are associated with overdensepockets of neutral gas which can be found at lower redshift. We produce mockobservations for a set of sightlines for the next generation of telescopes likethe ELT and SKA1-LOW, finding that given a sufficiently bright backgroundquasar, these telescopes will be able to detect both types of absorbersthroughout reionization.

Cospatial 21 cm and metal-line absorbers in the epoch of reionization -- I : Incidence and observability