  The nearby elliptical galaxy M87 contains one of the only two supermassive
black holes whose emission surrounding the event horizon has been imaged by the
Event Horizon Telescope (EHT). In 2018, more than two dozen multi-wavelength
(MWL) facilities (from radio to gamma-ray energies) took part in the second M87
EHT campaign. The goal of this extensive MWL campaign was to better understand
the physics of the accreting black hole M87*, the relationship between the
inflow and inner jets, and the high-energy particle acceleration. Understanding
the complex astrophysics is also a necessary first step towards performing
further tests of general relativity. The MWL campaign took place in April 2018,
overlapping with the EHT M87* observations. We present a new, contemporaneous
spectral energy distribution (SED) ranging from radio to very high energy (VHE)
gamma-rays, as well as details of the individual observations and light curves.
We also conduct phenomenological modelling to investigate the basic source
properties. We present the first VHE gamma-ray flare from M87 detected since
2010. The flux above 350 GeV has more than doubled within a period of about 36
hours. We find that the X-ray flux is enhanced by about a factor of two
compared to 2017, while the radio and millimetre core fluxes are consistent
between 2017 and 2018. We detect evidence for a monotonically increasing jet
position angle that corresponds to variations in the bright spot of the EHT
image. Our results show the value of continued MWL monitoring together with
precision imaging for addressing the origins of high-energy particle
acceleration. While we cannot currently pinpoint the precise location where
such acceleration takes place, the new VHE gamma-ray flare already presents a
challenge to simple one-zone leptonic emission model approaches, and emphasises
the need for combined image and spectral modelling.
