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Abstract We present analysis of one of the most extreme quasar outflows found to date in our survey of extremely high-velocity outflows (EHVOs). J164653.72+243942.2 (z_em ∼ 3.04) shows variable Civλλ1548,1551 absorption at speeds larger than 0.1c, accompanied by Siiv, Nv,and Lyα, and disappearing absorption at lower speeds. We perform absorption measurements using the apparent optical depth method and SimBAL. We find the absorption to be very broad (Δv ∼  35,100 km s⁻¹in the first epoch and 13,000 km s⁻¹in the second one) and fast (v_max ∼  –50,200 km s⁻¹and −49,000 km s⁻¹, respectively). We measure large column densities ( $\log N_{\mathrm{H}}>$21.6 (cm⁻²)) and are able to place distance estimates for the EHVO (5 ≲ R ≲ 28 pc) and the lower-velocity outflow (7 ≲ R ≲ 540 pc). We estimate a mass outflow rate for the EHVO to be ${\dot{M}}_{\mathrm{out}}\sim 50–290M_{\odot }{\mathrm{yr}}^{-1}$and a kinetic luminosity of $\log L_{\mathrm{KE}}\sim 46.5–47.2\left(\mathrm{erg}{\mathrm{s}}^{-1}\right)$in both epochs. The lower-velocity component has a mass outflow rate ${\dot{M}}_{\mathrm{out}}\sim 10–790M_{\odot }{\mathrm{yr}}^{-1}$and a kinetic luminosity of $\log L_{\mathrm{KE}}\sim 45.3–47.2\left(\mathrm{erg}{\mathrm{s}}^{-1}\right)$. We find that J164653.72+243942.2 is not an outlier among EHVO quasars in regard to its physical properties. While its column density is lower than typical BAL values, its higher outflow velocities drive most of the mass outflow rate and kinetic luminosity. These results emphasize the crucial role of EHVOs in powering quasar feedback, and failing to account for these outflows likely leads to underestimating the feedback impact on galaxies. 

We report the discovery of an extremely high-velocity outflow (EHVO) in the most luminous QSO (L_Bol∼ 2.29 × 10⁴⁸erg/s), named SMSS J2157-3602, atz = 4.692. Combined XSHOOTER and NIRES observations reveal that the EHVO reaches a maximum velocity ofv_max ∼ 0.13cand persists over rest-frame timescales of a few months up to one year. SMSS J2157-3602 also exhibits one of the highest balnicity index values discovered for an EHVO so far. In addition, the blueshifted CIV emission traces a high-velocity (v_CIV⁵⁰∼ 4660 km/s) outflow from the broad-line region (BLR). Thanks to an XMM-Newton observation, we were also able to reveal the X-ray weak nature of this QSO, which likely prevents the overionization of the innermost disk atmosphere and facilitates the efficient launch of the detected EHVO and BLR winds. The extraordinary luminosity of SMSS J2157-3602 and the extreme velocity of the EHVO make it a unique laboratory for testing active galactic nucleus (AGN) driven feedback under extreme conditions. Current uncertainties on the outflow’s location and column density strengthen the case for a dedicated follow-up, which will be essential to assess the full feedback potential of this remarkable quasar.