So idk if this is a super dumb question but I need it answered for fanfic purposes. Would a card like Forbidden Chalice negate Light and Darkness Dragon's effect?
If so after the effect of Forbidden Chalice ends would Light and Darkness Dragon return to its original attack, or would it return to the attack it had before activation (assuming its effect had been used previously). For this second question would this also work in the case Light/Dark End Dragon?
Well, I assume you mean to Chain it after another effect was activated, since only activating it by itself would cause it to be negated. If that's the case, you can negate it that way, yeah.
When a monster whose effect changes its own ATK or DEF, and only its only ATK or DEF has its effects negated, that change disappears, so it would change to what it had before its ATK or DEF changed.
At least this is how I remember it, but I am not sure if this is completely up to date.