  Observations and high-resolution hydrodynamical simulations indicate that
massive star clusters assemble hierarchically from sub-clusters with a
universal power-law cluster mass function. We study the consequences of such
assembly for the formation of intermediate-mass black holes (IMBHs) at low
metallicities ($Z=0.01\;Z_\mathrm{\odot}$) with our updated N-body code BIFROST
based on the hierarchical fourth-order forward integrator. BIFROST integrates
few-body systems using secular and regularized techniques including
post-Newtonian equations of motion up to order PN3.5 and gravitational-wave
recoil kicks for BHs. Single stellar evolution is treated using the fast
population synthesis code SEVN. We evolve three cluster assembly regions with
$N_\mathrm{tot} = 1.70$--$2.35 \times 10^6$ stars following a realistic IMF in
$\sim$1000 sub-clusters for $t=50$ Myr. IMBHs with masses up to $m_\bullet \sim
2200\:M_\mathrm{\odot}$ form rapidly mainly via the collapse of very massive
stars (VMSs) assembled through repeated collisions of massive stars followed by
growth through tidal disruption events and BH mergers. No IMBHs originate from
the stars in the initially most massive clusters. We explain this by
suppression of hard massive star binary formation at high velocity dispersions
and the competition between core collapse and massive star life-times. Later
the IMBHs form subsystems resulting in gravitational-wave BH-BH, IMBH-BH and
IMBH-IMBH mergers with a $m_\bullet\sim1000\:M_\mathrm{\odot}$
gravitational-wave detection being the observable prediction. Our simulations
indicate that the hierarchical formation of massive star clusters in metal poor
environments naturally results in formation of potential seeds for supermassive
black holes.
