The Black Map (ブラックマップ) is an accessory that can be equipped only by Vyse. Equipping this accessory increases the rate of random encounters in addition to effectively decreasing the chance of an enemy successfully running from battle to zero. This accessory also increases the chance of a first strike by 25%. The Black Map has a 4% chance of being dropped by most variants of the Looper monster line.
The modifier is actually a -99.9% chance of successfully running (similar to how the moonstone cannon has a hit% of 999 -- effectively never missing). This is an artifact of the programming of the game and only becomes relevent with enemies such as the Elcian that have absurdly high stats and can occasionally escape anyway. It is extremely rare for any other enemy to manage to bypass the effects of the Black Map, but it is possible.
Using the Black Map is an exceedingly good method for grinding out extra exp or money if need be.