After one of his many visits to the town of Horsehead, Billobi one day stumble upon an odd fellow in the surrounding forest. They were in fact two; one sturdy dwarf, and one thin and wiry boy, although the boy didn't say anything.
They were standing next to a worn and presumably very old iron fence, surrounding an even older stone platue. A narrow staircase was carved out of it, and descended steeply into the ground.
"Hail, traveller!" the dwarf growled with a big smile. "And what have we here, hmm? COMPETITION PERHAPS?"
He laughed with a deep voice and gave Billobi a friendly push.
"But seriously", said the dwarf and stopped laughing, "this ain't no place for wee lads like yourself. See this fence? Gives me the creeps! Just think about what horrors that lingers down there..."
He took out his axe and gently brushed the fence.
"'Tis one of them Horse Men-tombs, you know? Strange fellows, nobody's seen them for ages. Everyone's scared of them. Well, except for me and this young fellow! Ain't that right!"
The young boy responded with a strained smile. He looked nervous.
"That's the spirit!", the dwarf shouted. He turned to Billobi again, and said: "I picked him up in Horsehead, quiet little fellow, but someone's gotta help me carry all that gold!"
Billobi asked if they weren't afraid that there would be old spirits protecting the tomb.
"Naah", snorted the dwarf. "I've got my two lifesavers right here: me old axe..." - he kissed the small axe - "...and this ring from me father. It's magical, you see. It lets you part water, so you don't have to get wet. I hate water. You can't even control your body in it! I bet it's worth a fortune, magical an' all, but I ain't selling it. Never. You'd have to chop off my finger, or steal it in my sleep - either way, you'd end up with me axe in your back!"
The Tomb in Horsehead is a small underground adventure. The zipped archive contains the adventure as a single PDF, along with several maps in various sizes and formats. The original Word 2003-document can also be found inside the archive, if you want to tinker with it and change anything (or everything!).
I believe it's fairly system neutral, although I had Swords & Wizardry in mind when I wrote it.