The Behavior

When you enter y=40, time begins ticking. You will begin to hear cave sounds, and eventually, it will spawn. (The Lurker replaces nearby common mobs like zombies, so if the area is mob-proof, it's lurker-proof.)

The Cave Lurker has three behaviors when it does spawn, that it picks at random.

It can flee from you when you get close, so it's not always a threat.

It can move towards you while you aren't looking, like a Creaking. Don't let it get too close.

And it can of course, chase you.

While chasing you, if you can get it down to half health, it will be stunned for five seconds, creating an opportunity for escape.

This isn't just a fast guy kill you, it comes with drops. Using these drops, you can craft two powerful items.

The Drops

When stunned or killed, it will drop a few teeth, which can be used as an alternative to bones.

When killed only, it may drop an eye, which can be eaten, although it will give you momentary blindness, and it's not very filling, either.

Tooth Necklace

The Tooth Necklace is worn on the head, and while it provides no armor value, it will give you haste while standing on natural stones! (Stone, Granite, Diorite, Andesite, Tuff, Calcite, Deepslate and the ores.)

Deep Stew

Deep Stew is a powerful food item, having high food and saturation values. It also gives you haste II, night vision, strength I, speed I, and health boost II for almost five minutes.

And it will also drop assorted cave mob drops, and maybe even some ores if you're lucky.