If this card is Summoned: Target 1 face-up monster on the field; it loses 800 ATK.
Rulings
Individual Card FAQs
If the monster selected by "Dark Jeroid" is removed from the field by "Dimensionhole", or flipped face-down, or returned to the hand or Deck, or sent to the Graveyard, etc., the effect of "Dark Jeroid" is reset and the selected monster no longer has a lowered ATK.
If "Dark Jeroid" is the only monster on the field when it is Summoned, it reduces its own ATK.
The ATK cannot go below zero.
Netrep Rulings
Dark Jeroid's effect is Targeting.
If "Dark Jeroid" is the only face-up monster when "Dark Jeroid" is Summoned, "Dark Jeroid" must target itself for its effect.
"Dark Jeroid" only gets its effect when it is Normal Summoned, Flip Summoned, or Special Summoned. It does not get its effect when it is Flipped.
"Dark Jeroid" does not get its effect if Flipped as a result of an Effect or as a result of Battle.
If the monster affected by "Dark Jeroid" is turned face-down or leaves the field, the effect of "Dark Jeroid" disappears.
DARK|
Level 4|[ Fiend / Effect ]
ATK 1200|
DEF 1500