This card can only attack if this card is the only card on your side of the field and if you also have no cards in your hand. The effects of Effect Monsters that this card destroys are negated.
Rulings
Individual Card FAQs
If you control "Ultimate Obedient Fiend" and have no hand and no other monsters, but you have Spell or Trap Cards on the field, "Ultimate Obedient Fiend" cannot attack.
FIRE|
Level 10|[ Fiend / Effect ]
ATK 3500|
DEF 3000