A Wish spell is THE example of how a gamemaster in a Dungeons and Dragons game can mess with the twentieth-level half-dragon half-demon psionic ninja magic-user and the entire universe supporting him. As a ninth level spell whose mere casting ages the magic-user by five years, Wish is both a boon and a bane to those who would use it.

Essentially, the person who has the Wish can make one single-sentence demand of the universe, which must immediately fulfill it. However, the universe gets to interpret the demand however it wants to- sometimes, the better phrase would be "twist the demand." Merely saying "I wish for a thousand gold coins" is likely to teleport you into the middle of a massive coin pile that's still actually in the airtight vault of the gawd Throthgar, the gawd of Exquisite Pain- thus, anyone who wants to use a wish must word it as exactly as they can, lest the castle of the wish "I wish I had a castle!" appear twenty feet over their heads, and by falling the remainder of the way to the ground turn them into a pasty stain on the underside of the masonry. As a general rule, though, wishes used for heroic purposes (healing the party in a last stand against your enemies) are less often deliberately misinterpreted (if at all), while wishes used to further a character's greed are twisted to the letter- however, the GM is the final arbiter of what happens, so tread carefully in any case.

While wish is a ninth-level spell and thus only attainable on a regular basis by the most powerful wizard-types, items that convey a wish will often find themselves in the hands of a player-character- a Ring of Three Wishes, the infamous Deck of Many Things, a Luckblade, or even a scroll bearing the spell itself might make themselves available. The most reliable way to get the most out of a wish is as a last gasp, last resort, doomsday-button weapon- that way, even if the universe decides to screw you over, you can rest content in the fact that you would have been dead anyway.

Of course, if you've always wanted a keep of your own and don't have the patience to gather your own resources, manpower, and permission from the local lord, I won't stop you...