Durendal

If Durendal has a +1 counter on it from any effect, all cards with the Reaction type will have the conditional effect applied while defending Durendal’s attacks.

“Reaction cards” is any card with the Reaction type in its typebox. This includes attack and defense reactions. It does not affect cards that were added to the chain link by a defense reaction activated ability unless they also have the Reaction type. For example, Quickdodge Flexor becomes defending following the resolution of a Defense Reaction ability, but is not itself a card with a Reaction type.

An effect that applies a increase is not the same as a counter (e.g. targeting Durendal’s attack with Backside of the Blade will not meet Durendal’s condition)

Durendal’s second ability is functional regardless of how it attacks (e.g. And Again…)

Durendal’s second ability is a static-continuous effect that will continuously check if it has a +1 counter, not only on resolution of its activation. If Durendal loses all +1 counters, its second ability will no longer apply its effect and defending reaction cards will regain their value.