The Saint's Magic Power is Omnipotent (Light Novel) 1 by Yuka Tachibana, Yasuyuki Syuri (Illustrator)

A MISSION TO HEAL THE LAND.

On Earth, Sei was a twenty-something workaholic office worker, but in her new home of Salutania she's become the Holy Saint destined to save the world from darkness. Together with the handsome Captain Hawke and the magic-obsessed Magus Drewes, she's traveled to the far-off Klausner territory to try and purify the miasma plaguing their once-fertile land. Their first journey into the cursed wood was foiled by a pack of deadly slime monsters, but with reinforcements from the royal capital, they're prepared to return and make good on their quest.