You can Tribute this face-up card; take control of all face-up Level 3 or lower monsters your opponent currently controls.
Rulings
Individual Card FAQs
If the opponent has more Level 3 or lower monsters on the field than you have empty Monster Card Zones after you Tribute this card (the Zone it occupied is now empty), you choose which ones to gain control of and the rest are destroyed. If you choose to destroy "Griggle" or "Ameba" in this manner, their effects do not activate since they never came to your side of the field.
You cannot gain control of face-down monsters with this card's effect, so the opponent may chain "Book of Moon" selecting a Level 3 or lower monster and they will keep control of it.
You maintain control of the monsters until they are removed from the field.
Netrep Rulings
If there are more level 3 or lower monsters on the opponent's field than there is space on the player's field when "Possessed Dark Soul" is Tributed for its effect, the excess monsters are destroyed.
If a monster is reduced to Level 3 or lower by the effect of "Demotion," and control is taken by the effect of "Possessed Dark Soul," control will not return to the other player if "Demotion" becomes inactive.
DARK|
Level 3|[ Fiend / Effect ]
ATK 1200|
DEF 800