World legacy Key Ruling
Let's start from the beginning, &quot;World Legacy Key&quot;'s effect stated:

Negate any opponent's Trap effect that activates in the same column as a &quot;Mekk-Knight&quot; monster you control.

Assuming this Continuous effect already took place, your opponent activates &quot;Fiendish Chain&quot; with this activation condition:

Activate this card by targeting 1 Effect Monster on the field; ...

Which will now be explained by this article: Activate

A card or effect can only be activated if that card or effect meets its activation conditions.
The activation cost for a card or effect must be paid in order to activate a card or effect.
It is not possible to activate a card or effect if the effect would fail to do anything when it resolved.
It is not possible to activate a card or effect if it has no valid target.

Number 3 does NOT actually stop &quot;Fiendish Chain&quot;. It means fail to do anything by the game mechanic or does not change any game state if it resolved, not negated by card effects (for example, activating &quot;Raigeki&quot; or &quot;Dark Hole&quot; when there are no monsters in the field). It is perfectly fine to activate cards as long as the activation condition is met, regardless if it would be negated by even Continuous effects.
Additionally, &quot;Fiendish Chain&quot; continuously targets the targeted monster, therefore as soon as &quot;World Legacy Key&quot; is no longer active, &quot;Fiendish Chain&quot; will automatically negate the targeted monster at the time of activation.
Therefore, &quot;Fiendish Chain&quot; can be activated, but its effects are negated as long as &quot;World Legacy Key&quot; is active. &quot;Fiendish Chain&quot; will remain on the field.
According to Shinji's explanation below, &quot;Fiendish Chain&quot;'s Continuous Effect still negates according to this ruling about &quot;Skill Drain&quot;, so yeah, my bad. Kudos to him for finding the ruling in Japanese. Sometimes Konami make everything more convoluted than it should be.