CePtPb hosts a distorted kagome lattice of ${\mathrm{Ce}}^{3+}$ ions, providing a clean platform to investigate how reduced local symmetry and strong spin-orbit coupling reshape frustrated magnetism. Magnetization, specific heat, and magnetocaloric effect measurements, combined with a symmetry analysis of the single-ion anisotropy, demonstrate that the local $m2m$ site symmetry selects a nearly Ising-like Kramers doublet with easy axes lying within the $ab$ plane. This results in three distinct in-plane Ising directions and an overall easy-plane anisotropy. The low-energy magnetic response is well captured by a three-sublattice Ising model, which quantitatively reproduces the saturation magnetization for arbitrary in-plane field orientations, including [110] and $\left[1\overline{1}0\right]$, as well as the ratio of the field-induced critical fields. For $B\parallel \left[110\right]$, the phase diagram exhibits two quantum critical points at $B_{\mathrm{c}1}=0.25\mathrm{T}$ and $B_{\mathrm{c}2}=0.55\mathrm{T}$, arising from the sequential polarization of the three Ising sublattices. These results reveal that the system develops quasi-one-dimensional spin chains along the $c$ axis, emerging from the nominally three-dimensional crystal structure composed of stacked kagome layers, and illustrate how reduced local symmetry can drive effective dimensional reduction in rare-earth Ising magnets.

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• Exotic phases of matter
• Magnetic phase transitions
• Magnetism
• Phase diagrams
• Quantum phase transitions