Herin are a number of ways your magic item might come into being, apart from something you order from your local wizard.
A standard roleplaying environment: the tavern. This is another easy one to create in your own home, providing you have a large enough group of roleplaying friends. Let us assume your PC group is no larger than five, and that the rest of your friends can be NPCs. Let us also aim for an exceptionally seedy and unscrupulous city tavern.
In a good LARP it is desirable to separate the players out individually at some stage, because they are more suggestible and afraid when they are alone. Some form of individual challenge is probably the best way of doing this. One possible setting for this is a cave system, and it is probably one of the easiest to recreate realistically, and one of the most perfect for playing on fears of claustrophobia and darkness.
A wild species, vinus homophagus, more akin to sea-grape rather than the terrestrial variety, is not a monster despite its fanciful name. The grapes, a deep purple color when in bloom, and oozing dewdrops of perspiration, like the most prized and delectable of drinking wine grapes, do however deserve their moniker. Wine made from this fruit, is deadly to most humanoids, as is the raw berry if plucked and eaten from the vine. It is the unnatural chemical concoction found within the fruit’s tart skin, which gives the man-eating grape its name. The chemical stew found inside each berry, functions as a necrotic agent, the same as found in some species of venomous snakes.
The grapes literally eat their victims from the inside out, via cell death, melting and destroying the organs in quick succession.
The tribes of Pra-Oohk Crater, of the jungles of Ghlush are known to sell the fermented “wine” of this grape to merchants of distant lands. Sadly, the taste of the concoction is divine when first quaffed, and even worse, the man-eating grape wine will never detect as poisonous via mundane means, its horrid natures somehow masking all attempts. Luckily the man-eating grapes are extremely rare, and endemic to humid jungles.