If this card is discarded to the Graveyard by a card effect: Special Summon this card from the Graveyard to your opponent's side of the field in face-up Defense Position. When this card is Special Summoned by the effect of a "Dark World" card: Your opponent must discard 1 card.
These are the cards that has effects of discarding cards from your hand to the graveyard. This concept is different than discarding cards as a cost to trigger the card effect. Dark World Lightning has effects that discards 1 card from your hand. Into the Void’s effect will discard your entire hand during your End Phase, so if you discarded all your Dark World monsters, you can trigger their effects. Dragged down to the Grave has mandatory effects for both players to choose 1 card from each other’s hand and discard it, and draw 1 card. Trance Archfiend’s effect allows you to discard 1 Fiend-type monster once per turn, to boost its ATK by 500 for that turn.
There is something wrong with this card description. "... Special Summon this card from the Graveyard to your opponent's side of the field.... opponent must discard 1 card." End up its special summoned to your side and they discard another card themselves.
Opponent status should not change even if this card summoned over. I know how this card works but the description is wrong. You can compared this to fairy meteor crush. Equipped this to opponent card and when hit your def position card, the dmg is inflicted on their side.
The effect applies in favor of the controller of the card. That's how Fairy Meteor Crush does that. Yea it's your opponents monster but it's YOUR equip spell so you benefit from it. Guru is the same. Once is it's in your opponent's control YOU are now the opponent to that card.
Because Ceruli is discarded by a card effect (Trance Archfiend), and since it is Special Summoned to the opponent's field by the effect of a "Dark World" card (Ceruli itself), your opponent has to discard 1 card.