Forest and normal cavern. The game sets up this Hallow/Corruption/Crimson thing with purification powder, the Clentaminator, the Dryad's status updates and all that, but within what little narrative exists in Terraria, it's pointless. There's basically no reason for me to give a rat's excretion tube about the 'purity'. There's no reward for beating back the dangerous realms of the Hallow and Corruption/Crimson. Frankly, you're rewarded more by the game for overrunning the entire world with BOTH so you can conveniently keep access to everything you need with no drawbacks at all. The NPC's will even happily live in the Hallow, so the gameplay conceit of keeping the area around them livable becomes a sixty second operation of slinging some holy water about the place because the Hallow is an impenetrable wall.
Terraria: Otherworld operates from the assumption that the player in Terraria fails in their mission....the funny part is that because of how we play Terraria, that's actually the grim truth of like....practically everyone's world. We never restore the purity.....why would we? The hilarity isn't that we 'fail' in being unable to defeat the Moon Lord, it's that we basically just usurp the Hallow and Corruption/Crimson for our own purposes instead of ever purifying anything. Terraria 1 usually isn't the story of a conquering hero, it's the story of the rise of a supremely powerful mad monarch who spreads forth the terrifying realms of Hallow and Corruption because they fuel his/her power. You just think
you're the hero.
Think about it....the world you're dropped into when starting Terraria is actually balanced. If you leave it alone, nothing happens. It's fine. Everything you do constantly releases horrors and makes things worse
. When you start consuming life crystals to empower yourself (yeah, that sounds more messed up now, huh?) it reaches a point where a giant freaking eyeball tries to hunt you down and stop you at night. When you breach a slumbering Corruption/Crimson and smash open the receptacles of power you find in your greed for the power within, another massive beast shudders into life and tries to stop you. A cursed man exists to stop you breaching the EVIL DUNGEON, and you slaughter his powerful keeper so you can plunder it! You unleash and massively empower the Hallow and Corruption, you kill three further guardians who seek to stop your relentless quest for personal empowerment, you slaughter every invasion that tries to stop you.....did you ever notice that when you kill the 'lunatic' cultist things actually go to hell? A group of guys are standing around performing a magical ceremony and as long as they're doing that, nothing happens. Then you KILL them, shatter their ceremony, and INCARNATIONS OF UNIVERSAL POWER TRY TO STOP YOU. They fail, the Moon Lord finally tries to stop you, HE fails, and all that's left is the cackling psychotic that walked into a forbidden land on a quest for personal glory, hovering above the shattered hellscape that is his or her domain.
Terraria isn't 2D Minecraft, it's 2D Shadow of the Colossus, and you're not a hero.