  Magnet$/$superconductor hybrids (MSHs) hold the promise to host emergent
topological superconducting phases. Both one-dimensional (1D) and
two-dimensional (2D) magnetic systems in proximity to s-wave superconductors
have shown evidence of gapped topological superconductivity with zero-energy
end states and chiral edge modes. Recently, it was proposed that the bulk
transition-metal dichalcogenide 4Hb$-$TaS$_2$ is a gapless topological
nodal-point superconductor (TNPSC). However, there has been no experimental
realization of a TNPSC in a MSH system yet. Here we present the discovery of
TNPSC in antiferromagnetic (AFM) monolayers on top of an s-wave superconductor.
Our calculations show that the topological phase is driven by the AFM order,
resulting in the emergence of a gapless time-reversal invariant topological
superconducting state. Using low-temperature scanning tunneling microscopy we
observe a low-energy edge mode, which separates the topological phase from the
trivial one, at the boundaries of antiferromagnetic islands. As predicted by
the calculations, we find that the relative spectral weight of the edge mode
depends on the edge's atomic configuration. Our results establish the
combination of antiferromagnetism and superconductivity as a novel route to
design 2D topological quantum phases.
