Actually, Huck gets the ball rolling when he says that Tom's suggestion of holding hands and swearing to silence isn't enough. Huck says it's girlish to do it that way and only good for ordinary secrets. He suggests they use writing and blood.

Ever one to seize on an exciting idea, Tom is enthusiastic. Tom writes the following oath:

Huck Finn and Tom Sawyer swears they will keep mum about This and They wish They may Drop down dead in Their Tracks if They ever Tell and Rot.

Then, Tom takes out his sewing needle and has each of them prick his finger multiple times to get enough blood to form signatures. He shows Huck how to make an H and and F for his signature.

When Huck asks if this means they will never tell, Tom says yes, that this will force them to keep "mum" about what happened. He says they will drop dead if they ever break the oath.

Tom's imagination is one of his most appealing characteristics, and it's at its best when he's striving to do things as he thinks they should be done. Whenever possible, Tom strives to add ceremony and ritual and all the trappings of making whatever adventure he's involved with seem bigger-than-life.

Therefore, a mere promise between Tom and Huck that they won't give away the secret they share regarding their observations in the graveyard is not weighty enough for the situation. Huck suggests "there orter be writing 'bout a big thing like this. And blood." Tom seizes the suggestion and amplifies it. The oath he pens combines every impression of Robin Hood and pirates and all the other glorious figures he loved to imagine himself to be. The blood oath seals the solemn document, as required by any deep commitment entered into by such heroic and adventurous individuals.