Second Sun is an Evocation 8 global that costs 80 fire gems and requires a F8 caster.
"The caster creates a huge ball of fire in the sky. The Second Sun will always shine, day and night, resulting in severe effects across the entire world. Provinces will become hotter and drier every turn until the Second Sun is detroyed. This enchantment lasts until someone dispels it or the caster dies."
"As a sjgn of his coming reign <Pretender> has thrust a ball of fire into the sky to rival the light of the sun itself, slaying the shadows and the night, and exposing the base deeds of the wicked and faithless to the merciless glare of the burning eye of the <Pretender's Titles>.
This must stop, <Pretender>'s brazen display of power is beating down on your subjects and their lands, wilting their crops and drying out their rivers. How can your flock remain faithful when the sign of <Pretender> is hanging so unopposed in the sky, dispelling shade but causing the shadow of doubt to darken the minds of your faithful. Tear down this insult that has been hung above your head, dispel this merciless light and usher back the coolness of the night."
Second Sun increases Heat and Death scales across the whole map. It does not counteract Utterdark in any meaningful way. Theft of the Sun does not dispel it, but does counter it. While both of these enchantments are active, Sun Guides still get Awe from the Theft of the Sun and one still gets the gems from the global, but otherwise their effects balance each other out and nothing happens. Second Sun has no noticeable effect on Vengeful Waters, despite the spell descriptions.