Gains 1500 ATK while you have no cards in your hand nor in your Spell & Trap Zones.
Rulings
Individual Card FAQs
If you have only "Kuriboh" in your hand, and no Spell or Trap Cards on the field, and your "Theban Nightmare" battles with a card that has higher ATK/DEF, and you discard "Kuriboh", the ATK of "Theban Nightmare" is increased by 1500 AFTER damage calculation (but "Theban Nightmare" is still destroyed by damage calculation unless an effect like "Waboku" had been activated previously during the turn).
LIGHT|
Level 4|[ Fiend / Effect ]
ATK 1500|
DEF 500