Correlated magnon excitations in topologically nontrivial systems remain largely unexplored experimentally. Here, we report that 10% La substitution in localized-insulator $\mathrm{EuBiT}{\mathrm{e}}_3$ yields single crystals of $\mathrm{L}{\mathrm{a}}_{0.1}\mathrm{E}{\mathrm{u}}_{0.9}\mathrm{BiT}{\mathrm{e}}_3$ that simultaneously host antiferromagnetic order ($T_N=7\mathrm{K}$) and a topological Weyl-semimetallic state. Magnetic susceptibility and low-temperature specific-heat measurements reveal a field-dependent minimum at $T_M=3\mathrm{K}$ and a finite magnon gap that softens with increasing magnetic field-signatures of magnon gap formation. Electrical transport, including planar Hall measurements, follows semiclassical Boltzmann theory yet violates conventional Lorentz-force behavior, reflecting nontrivial Berry curvature effects. Angle-resolved photoemission spectroscopy directly visualizes linearly dispersive bulk bands crossing the Fermi level and isolated two-fold-degenerate Weyl nodes. These results establish $\mathrm{L}{\mathrm{a}}_{0.1}\mathrm{E}{\mathrm{u}}_{0.9}\mathrm{BiT}{\mathrm{e}}_3$ as a versatile platform for investigating the interplay between correlated magnetic excitations and topological Weyl phases.

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• Magnetism
• Magnons
• Topological materials