Cosmic reionization is the last major phase transition that our Universe goes through. In this milestone, the intergalactic medium transitions from primarily neutral, dark, and cold into mainly ionized, more luminous, and warmer. However, as exciting as the reionization process surely was, many aspects remain unknown, e.g. what are the sources of the ionizing photons? or what is the timeline of reionization? Current and upcoming telescopes (e.g. JWST and HERA/SKA) will directly probe cosmic reionization; nonetheless, interpretation of said measurements will strongly rely on modeling the surroundings of luminous objects (JWST) or on foreground removal (21 cm). In this talk, I will describe a promising novel method to constrain reionization through its impact on the post-reionization intergalactic medium using two promising probes of this era: the Lyman-alpha forest (e.g. DESI) and 21 cm line intensity mapping (e.g. PUMA/SKA). I will focus on both the astrophysics of reionization as a potential source of bias for cosmological analyses and as a promising new avenue to learn about the astrophysics that governs cosmic reionization and the standard cosmological model. Furthermore, I will showcase the potential of this effect to constrain the nature of dark matter via its exquisite small-scale sensitivity.