These are monsters that appear in the dirt layer in the Underground.
“The Spectral Elemental is a monster found only on the Console Version in the Underground Hallow...Spectral Elemental
“The Illuminant Slime is a monster that appears only in the Underground Hallow after enabling...Illuminant Slime
“The Skeleton Archer is a monster found in the underground biome, but most commonly in the...Skeleton Archer
“The Red Slime is a Slime variant found regularly in the Underground layer, usually closer to the...Red Slime