This is a Binding of Isaac: Afterbirth+ Mod for giving you a built-in D6 ability that doesn't take up your space item slot. It gets charged normally when you clear rooms. You can't use batteries to charge it.

The default keybind to activate this ability is 'T'. To change the keybind, go to the custom challenge called "Change Keybind For Built-In D6." It will prompt you to press a key; do so, and then quit the challenge.

A previous version of this mod required running an external program to handle the keybind, and also required the --luadebug isaac launch option, but none of that stuff is needed now.

Credits: Hyphen-ated, Brett824, Inschato