Forced: After a shadow effect dealt to Nazgul of Dol Guldur resolves, the engaged player must choose and discard 1 character he controls.
1. What does "forced" mean?
"Forced effects are initiated by specific occurrences
throughout a game, and they occur automatically,
whether the card’s controller wants them to or not." (p. 23)
2. If we cancel the shadow cards effect, is that shadow effect still considered resolved? If yes then there is no way to avoid this ability…
"Q. If I cancel the Shadow effect on a card dealt to the Nazgûl of Dol Guldur (CORE 102), is the effect still considered to have resolved, making me discard a character?
A: No. Resolving an effect means that the effect triggered and resolved to the fullest extent possible. Canceling the effect will prevent the Nazgûl of Dol Guldur’s ability from triggering, just as if the card had no Shadow effect to begin with." (FAQ 1.3, p. 7)