Altermagnetism has recently emerged as a distinct and fundamental class of magnetic order. Exploring its interplay with quantum phenomena such as unconventional superconductivity, density-wave instabilities, and many-body effects represents a compelling frontier. In this work, we theoretically confirm the presence of high-temperature metallic altermagnetism in ${\mathrm{KV}}_2{\mathrm{Se}}_2\mathrm{O}$. We demonstrate that the anomalous metal-insulator-metal transition arises from a Lifshitz transition associated with Fermi surface reconstruction. The previously reported spin-density wave gap is found to lie below the Fermi level in our study and is now recognized to be attributed to the V-shaped density of states, originating from orbital-selective and sublattice-resolved half-metal-like behavior on a specific vanadium atom. Furthermore, we identify the instability from the nesting of spin-momentum-locked one-dimensional Fermi surfaces, which induces the SDW state. These findings position ${\mathrm{KV}}_2{\mathrm{Se}}_2\mathrm{O}$ as a promising platform for investigating the interplay among altermagnetism, unconventional superconductivity, and density-wave order.

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• Nesting
• Spin density waves
• Altermagnets
• DFT+DMFT
• Many-body techniques