Something you should understand about comets.
Comets ars NOT giant snowballs/icy wanderers outgassing water.
Several missions to comets have found zero water on the surface. Deuterium in the atmosphere of the comet is not water on the surface.
Comets are rocks. Some small, some big. Some very big. As big as Venus.
Venus was born of Jupiter (Volokovski Worlds in Collision)
The giant comet that will become known as Venus get a capturing sling shot sending the comet into the inner solar system.
But if it's not out gassing water what is the tail?
Due to the electric nature of the solar system the comet now enters the suns electric field releasing the potential energy of the solar capacitor. Working much like a bug zapper. The energy is there but only released when something enters its field. The comet is actually drawing material towards it from the surrounding space not outgassing.
This is your dragon of the great cataclysm, ripping the atmosphere from the smaller Mars as it passes by and nearly doing the same to Earth.
The Noah event?
The Moses event?
Pace and love brother