Describe the circumstances that lead to the signing of the fatal bond by Antonio and its consequences in Shakespeare's The Merchant of Venice.

Ultimately, Antonio signs this bond to support his friend Bassanio, who is seeking to raise funds to marry the wealthy Portia. However, while Antonio would very much like to assist Bassanio in this endeavor, at this point in time, all of his money is tied up in his commercial endeavors.
For this reason, Antonio and Bassanio pay a visit to Shylock, a Jewish moneylender. Shylock and Antonio have a history of mutual enmity, and Shylock sees this as an opportunity to extract vengeance. As they negotiate the terms of the loan, Shylock makes his terms: should Antonio fail to repay his debt in time, then Shylock would be allowed to take a pound of flesh from Antonio's body.
Antonio does not take this threat seriously when he agrees to these terms, but as the play progresses, his commercial endeavors fail, leaving him without the money to repay the debt. Meanwhile, Shylock remains intent on taking his due pound of flesh. It is only through Portia's ingenuity that the tables are turned, and Antonio escapes unharmed.

Antonio's dear friend Bassanio needs money to help him woo Portia, who Bassanio loves and wants to marry. Antonio is a wealthy merchant, but right now all his ships are at sea, bringing back the merchandise that Antonio will then sell at a handsome profit. Antonio has no cash at the moment to lend his friend, but Antonio wants to help him.
Antonio therefore approaches Shylock, a Jewish moneylender. We quickly learn there is bad blood between the two. Shylock, not surprisingly, resents Antonio's anti-Semitism and insults. Antonio, in turn, dislikes Shylock.
Shylock makes a bargain with Antonio to lend him the money. However, if it is not repaid within three months, Shylock will have the right to take a pound of Antonio's flesh from any part of his body. Shylock eventually plans to take it from his heart, which would, of course, kill Antonio. In those days, before modern, sophisticated ideas of sanitation or any antibiotics, cutting off that much flesh anywhere would probably be a death sentence. In any case, without painkillers, it would be a painful operation.
Antonio, however, is initially not worried. He has more than one ship out at sea and figures that at least one will return within three months.