Athena synergies in the multi-messenger and transient universe

In this paper we explore the scientific synergies between Athena and some of key multi-messenger facilities that should be operative concurrently with Athena. These include LIGO A+, Advanced Virgo+ future detectors for ground-based observation gravitational waves (GW), LISA space-based observations GW, IceCube KM3NeT neutrino observations, CTA very high energy observations. science themes encompass pressing issues in astrophysics, cosmology fundamental physics such as: central engine jet compact binary mergers, accretion processes Super-Massive Binary Black Holes (SMBBHs) stellar binaries, equation state neutron stars, cosmic accelerators origin Cosmic Rays (CRs), intermediate high-Z elements Universe, distance scale tests General Relativity Standard Model. Observational strategies implementing identified topics are also discussed. A significant part sources targeted by is transient nature. We have thus discussed synergy \textsl{Athena} wide-field high-energy facilities, taking THESEUS as a case study discovery. This discussion covers all goals rely on follow-up transients external observatories, includes not based search missing baryons or early star populations metal enrichment at dawn Gamma-Ray Bursts (GRBs).