Beware

It all started as a normal ordinary day when I was eating my sloppy muesli, on my crumpled old table and shrivelled up chair with Star ,my pet dog when suddenly there was a tapping on the door.
"Who could that be at this time in the morning?" I asked Star.(I knew she couldn't answer but she was my only family left since the tragic day)
As I turned the door knob it creaked loudly.
I looked outside and nobody was there so I shut it thinking no more of it but just as I turned around something touched my shoulder.....

Not daring to turn round I looked at Star. She ran into the living room worriedly. Trembling with fear I turned around that's when I saw it the creature.
I couldn't move, i felt stuck.
It had green skin, black eyes matching his black teeth dripping with purple blood.
"Beware" it said in a low monstrous voice.
"Of-of what" I said.
"Of the Gold Gem" it said.
But before I managed to ask anything it rushed out the door.
Faintly I heard in the distance "Remember beware of the Gold Gem"
I shut the door slowly and then went to sit next to Star we looked at each other we were both thinking the same thing.
Who was this creature and what did it mean?