This deck works around a few cards with life gaining effects and a powerful beatdown component, disregarding my own life points, to deal damage quickly and effectively

The damage comes mostly from an early tormented hero and the rakdos crew.

Pain seer and underworld connections keep my hand with some threats, allowing to keep up mid/lategame.

Finishing the curve are the bestow costs of Herald of torment and Spiteful Returned, but both of them are rather efficient as standalone cards, as Returned brings quite the damage even if the opponent blocks, and Herald brings evasion to the table.

Whip of erebos allows to gain some of the life spent back, to keep up for the lategame, and to whip back threats from the grave for a last attack, also allows us to sink mana later in the game

Nykthos is there to give a decent devotion boost, usually allowing to dump the whole hand into the field in one turn.

The deck seems to be a bit faster than mono black devotion, and to be fair, a bit cheaper.

The sideboard allows the deck to go into a more controlling side, if needed.