  We report on the discovery of a mysterious ultra-steep spectrum (USS)
synchrotron source in the galaxy cluster Abell 2877. We have observed the
source with the Murchison Widefield Array at five frequencies across 72-231 MHz
and have found the source to exhibit strong spectral curvature over this range
as well the steepest known spectra of a synchrotron cluster source, with a
spectral index across the central three frequency bands of $\alpha =
-5.97^{+0.40}_{-0.48}$. Higher frequency radio observations, including a deep
observation with the Australia Telescope Compact Array, fail to detect any of
the extended diffuse emission. The source is approximately 370 kpc wide and
bears an uncanny resemblance to a jellyfish with two peaks of emission and long
tentacles descending south towards the cluster centre. Whilst the `USS
Jellyfish' defies easy classification, we here propose that the phenomenon is
caused by the reacceleration and compression of multiple aged electron
populations from historic active galactic nucleus (AGN) activity, so-called
`radio phoenix', by an as yet undetected weak cluster-scale mechanism. The USS
Jellyfish adds to a growing number of radio phoenix in cool-core clusters with
unknown reacceleration mechanisms; as the first example of a polyphoenix,
however, this implies the mechanism is on the scale of the cluster itself.
Indeed, we show that in simulations, emission akin to the USS Jellyfish can be
produced as a short-lived, transient phase in the evolution of multiple
interacting AGN remnants when subject to weak external shocks.
