The encounter of the unknown entity

Me and my friends are playing in a private Minecraft server.I will call my friends A,B,C respectively.We are in a discord voice channel.

We all go for a mining trip.The Minecraft moon is rising.

A:Hey guys,I found a dungeon,check it out.

A showed us the coordinates of the dungeon and we all go to check out that dungeon.Before we arrived,A said one thing before he leave the voice channel and gone offline.

A:What was that?

A leave the voice chat and didn't join back.

I told B and C to continue playing while I phone A.I left the server.I phoned A 3 times,no one answered.Suddenly I heard that B and C are talking about something.

B:Arrived.I will dig in.

B and C checked out the dungeon.This dungeon is weird.

C:This dungeon have no mob spawner,only a chest.

B:What is inside the chest?Ima check it out.

C:Ok.

B:It contains two music disc:13 and 11.A name tag and a written book.

C:What is the title of a book?

B:The title is "Hello Miners"

C:This is weird,written books aren't naturally generate in a normal Minecraft world.Read the book.

The book says:

(Welcome to my cave,so glad to see you visitors.Lets have some fun here.)

C:Hmm... looks like there is other players in this world.But when I press tab I only see us.

I joined back the server.I saw a message which says:??? has joined the game.I pressed tab and saw a entity with no name and with a missing texture skin.B and C noticed that.They say something in the discord chat.

B:Why there is a player that we don't know in this private server?

C:Idk but...

B and C left the discord chat.

I saw the entity in front of me and it disappeared after a few seconds of awkward staring.I saw the forest outside the cave is burning.

??? has left the game.

I thinked that the dungeon is ominous,so I didn't check it out.And A.B and C went missing,I didn't see and heard them again.After the encounter,I went to the Minecraft house that we built.Everything is normal except there is a sign placed inside the house.The sign says:"You are lucky,don't let me see you again.Or you will be in danger."

I leave the server because I am a little bit scared.

Two weeks after the encounter,I decided to join back the server to discover the secrets hidden there.But I cannot connect to that server anymore.