Athena will be a powerful, general-purpose open X-ray observatory with an unprecedented combination of collecting area, survey capabilities and energy resolution, that will allow the astronomical community to address a wide range of astrophysical topics. The observatory will lead the quest of the following key questions, among others:

• Ascertain the nature of the primary source of high-energy radiation in stellar-mass and supermassive accreting black holes (Active Galactic Nuclei, AGN), and its connection with accretion and ejection mechanisms close to the event horizon.
• Determine the mechanism(s) regulating the cosmological co-evolution of accreting black holes and their host galaxies.
• Measure the space density of the AGN that dominate the supermassive black hole growth, reaching combinations of luminosity and obscuration hitherto unexplored.
• Constrain the kinematics of hot gas and metals in massive halos (galaxy clusters and groups).
• Map the properties of the most common baryonic reservoirs in the Universe, and probe their evolution and connection to the cosmic web.
• Constrain supernova explosion mechanisms by determining the 3-dimensional kinematics, ionisation state and abundances in young remnants probing the physics of the enrichment and heating of our Galaxy's interstellar medium.
• Provide novel and unprecedented constraints on the equation of the state of neutron stars exploring the behaviour of matter under extreme conditions of density and magnetic fields.
• Study star-planet interactions through the stellar magnetic activity in exoplanet-hosting systems.

Aside from these topics, Athena's singular and outstanding capabilities as an open observatory are expected to impact all Astrophysics fields profoundly. This will not happen in isolation, but taking full advantage of Athena's synergies with the impressive set of multi-messengers astronomical facilities in exploitation in the second half of the 2030s.