Then if this card was already on the field for more than a turn, use "Book of Moon" to flip it face-down and flip it back up and send the "Blue-Eyes White Dragon" to your Graveyard to banish another card from your opponent's Graveyard.
This card's effect can target any card in either Graveyard and is not limited to monsters. This can prevent your opponent from using Spells or Traps that activate in the Graveyard like "Mischief of the Yokai", can be retrieved from the Graveyard like "Iron Core of Koa'ki Meiru", or clearing your own Graveyard of Spells for cards like "D.D. Borderline".
You can actually use this card's effect to dump DARK dragons to your graveyard so you can summon "Dark Armed Dragon" easier. If you have 1 DARK monster in your grave and "DAD" on your hand use this "Dragon's" effect to dump 2 DARK Dragons to Graveyard (1 from your hand and deck).
You can also use this card to banish a "DARK" monster from your graveyard if you have more than three in your graveyard. However, be sure not to send a DARK Dragon to your graveyard when doing so, as this will stop you from being able to summon "Dark Armed Dragon".
This card can replace "Dragon Ravine" in Disaster Dragon Decks, because this card also sends 1 Dragon from your hand and deck to the Graveyard.
Compared to "Dragon Ravine", this card can be a potential Dead Draw (due to this card's Level 5). Rather than replace it, use "Dragon Ravine" to put this card in the Graveyard, allowing "Red-Eyes Darkness Metal Dragon" to revive this card later. It is recommended ONLY activate this card's effect if you don't have "Dragon Ravine" at the moment (for example, if your opponent destroy it).
While it can seems a Dead Draw, being a level five is quite a help since it can special summon itself if you banish one LIGHT and one DARK monster, being able to Xyz Summon "Vice Dragon" (special summoned from it's own effect), or then become a synchro material with to Special Summon "Stardust Dragon", "Red Dragon Archfiend" or "Dark End Dragon".