What is "boktai"

is an action-aventure video game released on the Game Boy Advance in 2003 by Konami.

The player takes the role of Django, a vampire hunter, who uses a weapon called the "Gun Del Sol" (Solar Gun), that fires bolts of sunlight at enemies. Boktai's game cartridge includes a photometric light sensor that measures the amount of sunlight exposed to it. In order to charge the in-game weapon, the player must actually take their Game Boy Advance outside in the daytime (as verified by the light sensor). If the player's gun runs out of light reserves and there is no sunlight available, then the player must avoid enemies.