Den of the Spider (Red)

“When this defends” is an event-based trigger that triggers when Den of the Spider resolves on the stack and becomes a defending card (i..e defends the attack) against an attack with greater than its base. The power of the attack is checked at the time this resolves and becomes a defending card (i.e. defends the attack). If the trigger condition is met only after this resolves, then this does not trigger. When Den of the Spider’s triggered effect resolves, you will mark the attacking hero (if they aren’t already marked).

If there is no attacking hero (e.g. this is defending against an ally attack), nothing happens when the trigger resolves.