We present fine momentum space resolution resonant elastic x-ray scattering measurements of the magnetic structure of the metallic antiferromagnet ${\mathrm{CoNb}}_3{\mathrm{S}}_6$. Using circular dichroism and full linear polarization analysis of the magnetic scattering, we reveal a noncoplanar double-$\mathit{Q}$ $\left(2Q\right)$ order that is comprised of a noncollinear commensurate component and a long-wavelength incommensurate helical component. This $2Q$ magnetic structure exhibits a staggered scalar spin chirality that forms a modulated stripe like pattern with no uniform component. Measurements of the magnetic structure across many samples reveal a complex domain pattern associated with the magnetic ordering and suggest a lowering of the structural symmetry in ${\mathrm{CoNb}}_3{\mathrm{S}}_6$. We present a symmetry analysis demonstrating that the observed $2Q$ magnetic order breaks all necessary symmetries to enable an anomalous Hall effect and further show how this magnetic order can be naturally explained by four-spin exchange interactions in a metallic magnet. The identification of the magnetic order, associated symmetry breaking, and explanation of its origin provides insight into the mechanism of the unconventional magnetotransport phenomena in ${\mathrm{CoNb}}_3{\mathrm{S}}_6$ and thus can help to determine potential routes for realizing novel electronic phenomena in metallic antiferromagnets.

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• Frustrated magnetism
• Helicoidal magnetic texture
• Magnetic order
• Magnetism
• Spin texture
• Chiral magnets
• Resonant elastic x-ray scattering
• X-ray magnetic circular dichroism
• X-ray resonant magnetic scattering