Impure Monster (イムピュルモンスター Imupyuru Monsutā) is a kind of Monster Card. They appear in Yu-Gi-Oh! D-ZERO and Yu-Gi-Oh! D-ZERO II. Impure Monsters were introduced in Starter Deck D-ZERO, though their functions and use have changed since Starter Deck D-ZERO 2017 Q2. These cards resided in the Extra Deck starting from Starter Deck D-ZERO until Love Unbound. In Structure Deck S: Revival of Hiro, they were relocated to have their own Deck Zone, the Impure Deck. As they cannot exist in the hand or Main Deck, and now (except for Pendulum Monsters) Extra Deck, any card effects that would place them in any of those locations would otherwise place them in the Impure Deck.
As of the Master Rule 4, Impure Monsters, except Impure Pendulum Monsters that were then Special Summoned from the Extra Deck, must be Special Summoned to a Main Monster Zone, as they are not treated as being Special Summoned from the Extra Deck.
These cards are somewhat similar tos in appearance. The upper half is whatever colour of the kind of secondary monster it is (yellow for Normal Monster for example), but with a crimson bottom.
Unlike most other monsters, non-Xyz Impure Monsters' Levels use negative numbers to determine their power range. Because they're Negative Levels, they are treated as having Levels below 0. Likewise, certain effects that would affect a monster and/or its Level will have the opposite effect (e.g. if "" would banish an Impure Monster with its effect, it would instead lose ATK and DEF because the monster's original Level is a negative number). The lower the negative number, the more powerful the monster is.
Like Xyz Monsters, non-Xyz Impure Monsters have their Materials neatly stacked underneath the monster in face-up Attack Position. Most Impure Monsters require the player to "delete" their Materials to activate effects, making them limited in use, much like detaching for Xyz Monsters.
Xyz Impure Monsters instead use Xyz Material if the Impure Material Xyz Monster has Xyz Materials. The Xyz Monster in this case is flipped face-down Attack Position to distinguish it from its Xyz Materials. Starting from Starter Deck 2017: Reloaded-onwards, Xyz Monsters lose their Xyz Materials when used as Material. Likewise, they will no longer be flipped face-down.
In Yu-Gi-Oh! D-ZERO II, they appear as Truth-Seeking Balls that float behind the monster (in a sigmoidal formation if there are more than 2).