  Molecular cold gas and star formation have been observed at centers of
cool-core clusters, albeit at a level much smaller than expected from the
classic cooling model. Feedback from the supermassive black hole is likely to
have prevented hot gas from cooling. However, the exact cooling and heating
processes are poorly understood. The missing key piece is the link between the
hot gas ($10^7$\,K) and cold gas ($10^3$\,K). Using the extreme ultraviolet
spectrometer onboard {\sl Hisaki}, we explore a distant galaxy cluster, RCS2
J232727.6-020437, one of the most massive cool-core clusters with a cooling
rate of $400$\,M$_{\odot}$\,yr$^{-1}$. We aim to detect gas at intermediate
temperatures ($3\times10^4$\,K) emitting He I$\alpha$ and He I$\beta$ at rest
wavelengths of 58.4 nm and 53.7 nm, respectively. Our target resides at
$z=0.6986$, for which these He I lines shift away from the absorption of the
Galaxy. Our findings show that the amount of $10^{4-5}$\,K gas at the center of
this cluster is smaller than expected if cooling there was uninhibited, which
demonstrates that feedback both operates and is efficient for massive clusters
at these epochs.
