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The origin of radio relics is usually explained via diffusive shockacceleration (DSA) or re-acceleration of electrons at/from merger shocks ingalaxy clusters. The case of acceleration is challenged by the low predictedefficiency of low-Mach number merger shocks, unable to explain the powerobserved in most radio relics. In this Letter we present the discovery of a newgiant radio relic around the galaxy cluster Abell 2249 ($z=0.0838$) usingLOFAR. It is special since it has the lowest surface brightness of all knownradio relics. We study its radio and X-ray properties combinig LOFAR data withuGMRT, JVLA and XMM. This object has a total power of $L_{1.4\rm GHz}=4.1\pm0.8 \times 10^{23}$ W Hz$^{-1}$ and integrated spectral index $\alpha = 1.15\pm0.23$. We infer for this radio relic a lower bound on the magnetisation of$B\geq 0.4\, \mu$G, a shock Mach number of $\mathcal{M}\approx 3.79$, and a lowacceleration efficiency consistent with DSA. This result suggests that amissing population of relics may become visible thanks to the unprecedentedsensitivity of the new generation of radio telescopes.

Discovering the most elusive radio relic in the sky: Diffuse Shock Acceleration caught in the act?