Here is what The Rift adds:

Rift Stalker

The rift stalker is a charcoal colored quadrupedal creature that spawns in the End. When they see you they run over to you and they bite you which teleports up a couple blocks and back a couple blocks which could nock you off the islands. If you kill one you get the advancement Abyssal Hunter which rewards 500 xp, you can also get up to 2 rift shards by killing them.

Rift Shards

Rift Shards are purple gems that you get by killing Rift Stalkers. You can use them to craft Rift Pocket Watches and Rift Cleavers.

Rift Pocket Watch

You get the Rift Pocket watch by puting these things in a crafting table in this shape: 

Rift Shard  Rift Shard  Rift Shard

Rift Shard  Compass Rift Shard

Rift Shard  Rift Shard  Rift Shard

If you right click with it in you're hand you teleport 8 blocks in whatever direction you're facing.

Rift Cleaver

The Rift Cleaver is a bright magenta sword. If you hit something with it they will get teleported 3 blocks in the air. You can craft it like this:

Rift Shard Rift Shard Nothing

Rift Shard Netherite Sword Rift Shard

Nothing Rift Shard Rift Shard