What happens if Gates of the Dark World discarded Grapha to destroy Evilswarm Ophion but then Eradicator Epidemic Virus is chained and wants to tribute Ophion in Yu-Gi-Oh?

Answer 1

1. If Eradicator was chained to Grapha's eff targeting Ophion, then Eradicator will resolve first, tributing Ophion and destroying 1500 or less ATK monsters. Grapha's effect will then resolve. Since the intended target, Ophion, is not on the field anymore, Grapha's effect resolves with nothing happening.
2. If Eradicator was chained to Gate's effect, then Eradictor will do its effect and since Grapha's effect will only activate after the card effect discarding it resolves, Grapha' effect will not activate. So after Eradictor and Gate resolve, Grapha will select a target, which can't be Ophion as it's no longer on the field and destroy it.