This card is the bane of any Deck which forces the opponent to discard, like another "Dark World" Deck, or a Mill Deck. Thus, this card may be worth putting in the Side Deck.
This card is very difficult to Special Summon by its own effect ("Dark Deal" is not especially reliable) but it can still be sent to the Graveyard and easily revived with "Gateway to Dark World".
If you run this monster, make sure you've got a combo to get its effect activated easily. Try a few copies of "Dark Deal" to ensure that. In addition, this monster is a great side-deck choice against "X-Saber" and "Ninja" Decks which rely on the effect of "Goe Goe the Gallant Ninja".
Use "Fabled Dianaira" to discard "Reign-Beaux, Overlord of Dark World", as the effect of "Fabled Dianaira" is the same as "Dark Deal" without the element of chance or loss in Life Points.