If all cards in your hand are "Dark World" monsters whose effect activate when they are discarded from the hand to the Graveyard by a card effect, you'll get two benefits for the price of one. Not only do you get the effect of the card (whether it'd be Special Summoning them to the field or drawing one card), you have negated your opponent's card effect, all for 1000 Life Points.
Use this card in conjunction with "Neko Mane King". If this card is selected, your opponent's turn will end immediately. This is most effective when "Neko Mane King" is the only card in your hand.
If you can, try to set/Summon all cards that do not have a Secondary "Dark World" effect, so that you will get a devastating effect no matter what.
This is one of the best card to be used to Summon "Reign-Beaux, Overlord of Dark World", which is the hardest "Dark World" boss to Summon since it needs to be discarded by your opponent. By using "Dark Deal", you basically force your opponent to discard "Reign-Beaux". Of course, do this only when "Reign-Beaux" is the only card in your hand, since the discarded card is randomly selected.