For this achievement, you will need to earn 16 favor from Kali to receive the Kapala. To earn favor, you need to sacrifice stunned or dead bodies by placing them on Kali's altar, which has the possibility to be generated on any level. The altar is found at the bottom of the statues of Kali, which are the 8-armed women. Favor carries over to each stage, but if you anger Kali by destroying her altar, you will be unable to earn any more favor and you will have to start a new game.
For a chart with favor values, click here.
Perhaps the quickest and easiest way to obtain this, is to start a co-op game. You can sacrifice the second player to earn favor and then one damsel will be enough to receive the Kapala. You could also sacrifice the second player, then kill yourself and have your dead body land on the altar.
Once you have earned 16 favor and the Kapala has appeared on the altar, the achievement will unlock.