Cosmic evolution of BH 2, the second most massive BH at redshift ∼3.5, which reaches a mass M_• ∼ 10⁸ M_⊙ at the end of the simulation. The BH mass M_• (top panel, solid line), galaxy mass M_* (dashed line), the spin magnitude |a|, the cosine of the angle θ between the spin direction j_• and the gas angular momentum j_gas, the z-components of j_• and j_gas, and the Eddington ratio f_Edd, are plotted against the cosmological redshift. Vertical red dotted lines indicate the time of a numerical BH merger, with the vertical position of the crosses showing its mass ratio 0 < q ≤ 1.