
Take a hit. Get clues. This card is ridiculously good (compare it to, say, Intel Report). But there is a couple mechanics to keep in mind of.
1) It has to be damage! A spooky type of enemy that deals no damage wont let you trigger this!
2) You cannot prevent the damage, it has to be at least 1 damage not negated and not resisted, so a Flesh Ward for example blocking a 1 damage attack would render this unplayable.
3) It has to be an attack, so any effects enemies might cause that deal damage dont count, only actual attacks.
So yeah, with those conditions cleared, have fun with your clues!
Obviously you ought to keep some horror healing handy to recover some of the horror (and damage) you will inevitably take for using this card.