After Aleph tracks down Hanada, per Madam's request, Hanada tries to escape to the Expanse using the Summon Dolls, but due to an error in the ritual Mercurius is summoned instead. Mercurius attacks Aleph after killing Hanada.
"The Roman god of travelers and thieves. A diverse god, he is seen as a symbol of human unconscious and the mental world. He is equated with the Philosopher's Stone, the ultimate mystery in the field of alchemy."
—Persona 5 compendium
Mercurius is the ultimate Persona of Morgana, attained through a Second Awakening when his Confidant automatically maxes out on the midnight of December 23rd.
Due to the nature of the Magician Arcana Confidant, It is impossible to use Mercurius or view its compendium entry before December 24th in the original Persona 5, the last day where combat is available. This applies even on a New Game Plus setting where Personas that can only be fused on that day such as Yoshitsune or Vishnu could be used outside there if they are already registered. In Royal, Hereward possesses this trait instead of Mercurius.
Tiny Phys attack. High critical rate but low accuracy. (1 enemy)
Slight HP restore. (Party)
Light Wind attack. (All enemies)
Medium Wind attack. (1 enemy)
Moderate HP restore. (Party)
3-6 medium Phys attacks to random targets. (All enemies) Stronger in Boost.
Medium Phys attack. High critical rate but low accuracy. (1 enemy)
Prevent Agility Bind.
Heavy Wind attack. (1 enemy)
Revive and restore all HP. (1 ally)
Heavy Wind attack. (All enemies)
Full HP restore. (Party)
Mercurius as he appears in Persona 5
Concept art in Persona 5
☿ Planet Mercury's symbol
Kazuma Kaneko's art of Mercurius has his tentacles tied to the symbols of a moon, a sun and a cross which are the components of the planet Mercury's symbol, otherwise the stylized version of Hermes' caduceus. Shigenori Soejima's design for Persona 5 simply has the astronomical symbol hanging from the zip slider at the chest opening of Mercurius' jumpsuit.