A very scary Minecraft 1.19 snapshot, Mojang say, with the introduction of a new biome along with a new mob - and you can test it all out right now.
Exploring underground is about to get a lot more interesting, with the addition of the Deep Dark biome. Not only will you find Ancient City structures underground, you'll also come face to face with the Warden - a new mob. The Warden comes with a new Darkness mob effect, which is a major new mechanic. When nearby you'll be afflicted with it, which lowers the gamma down to a "Moody" level, making torches more important than ever. This can be tweaked in the Accessibility settings though.
Careful, they sense vibrations and can smell you.
They also added Sculk, Sculk Veins, Sculk Catalyst and Sculk Shrieker blocks. Some of these blocks will alert Wardens if you go too near, or step on them. This all sounds quite exciting actually, Minecraft is really starting to add in more fun things while exploring.
More info in the blog post.