Illuminant Slime

They fill the underground Hallow once a world turns, standing in for the black and mother slimes that used to hold those caves. No summon, no schedule, this is simply what the cave layer becomes. They hit for considerably more than the blobs you learned on, and the first one usually catches you at a full run while you are chasing crystal shards.

They light themselves faintly and drag a long pink smear behind them, the same after image the Chaos Elemental and the Illuminant Bat leave. In an unlit cavern that is a real warning before anything touches you. Watch for the trail coming around a corner rather than the slime itself.

• Early in Hardmode lava does the job for free. Grapple to a ledge above a pool and let them hop in after you.
• Royal Gel makes them ignore you entirely, which turns the Hallow cave layer into somewhere you can mine in peace. Worth an accessory slot if you plan to be down there for hours.