  We consider inflationary models with multiple spectator axions coupled to
dark gauge sectors via Chern-Simons (CS) terms. The energy injection into
Abelian gauge fields from the axions engenders a multi-peak profile for scalar
and tensor spectra. We highlight the constraining power of CMB spectral
distortions on the scalar signal and discuss the conditions under which
spectator sectors can account for the recently observed stochastic
gravitational wave (GW) background in the nHz range. Given the tantalizing
prospect of a multi-peak ``GW forest'' spanning several decades in frequency,
we elaborate on possible ultraviolet origins of the spectator models from Type
IIB orientifolds. String compactifications generically produce a multitude of
axions, the ``Axiverse'', from dimensional reduction of p-form gauge fields.
The CS coupling of such axions to dark gauge fields in the worldvolume theory
of D7-branes can be tuned via multiple brane wrappings and/or quantized gauge
field strengths. If string axions coupled to Abelian gauge fields undergo
slow-roll during inflation, they produce GW signals with peaked frequency
distribution whose magnitude depends on the details of the compactification. We
discuss the restrictions on spectator models from consistency and control
requirements of the string compactification and thereby motivate models that
may live in the string landscape as opposed to the swampland.
