All monsters only spawn in the Defiled Lands and aren't affected by the various nefast plants

See Book Wyrm.

Scuttlers are aggressive spider like creatures that will try to circle their target while attacking them. Unlike other monsters, they can spawn in broad light, albeit with at a reduced rate.

Drops String and Scuttler Eyes.

Shamblers are tall undead creatures of some sort. While it's very easy to avoid them due to their slow speed, they are immune to projectiles and knockback, and their hits will deal a lot of damage and knock their victim far away, inflicting several seconds of Slowness, Weakness and Mining Fatigue.

Shamblers will instantly disappear when exposed to sunlight.

Drops Black Hearts.

Twisted Shamblers are more aggressive undead creatures, they only spawn underground where they replace standard Shamblers. They share all their characteristics with the standard ones, except that they move much faster.

Drops Black Hearts.

Hosts are zombie-like creatures. Whenever they take damage, they will spawn multiple Defiled Slimes.

Drops Rotten Flesh and Foul Slime.

Defiled Slimes are pink slimes spawned whenever Hosts take damage. They are faster and more damaging than their green counterparts. Just like vanilla slimes, bigger Defiled Slimes will split upon death.

Drops Foul Slime.