A fanciful notation, commonly attributed to historical maps, held to indicate either the belief that unknown dangers exist in a certain location on the map, or that actual dragons can be found there.
By extension, used to indicate that which is unknown or uncertain, particularly if it seems to carry some type of risk.
Source: en.wiktionary.orgHere be dragons to be slain, here be rich rewards to gain; If we perish in the seeking, why, how small a thing is death! Dorothy L. Sayers
Monsters Beware Here Be Dragons. Laurell K. Hamilton