This is a very situational card. Usually, it's a testless clue that costs you a horror, an action, and a resource. Which isn't all that efficient a way to gain clues, unless you have some way to lessen the costs. However, if you have some way to make the costs less severe, it's a useful card for any investigator who doesn't have great investigating skills. The best use of it I've seen was a Yorick deck where he had ways of dealing with horror (Meat Cleaver and Cherished Keepsakes being the primary ones), and with his ability, he could put the raven into play without taking an action.
Discard Mysterious Raven and take 1 horror: Discover 1 clue at your location.
of a demon's that is dreaming..."
- Edgar Allan Poe, "The Raven"
Compare to Art Student: the Raven costs 1 less, but also costs a Horror to use, and the Student can soak a Damage and 2 Horror after finding a clue instead of 1 Horror instead of a clue.
Compare to Stray Cat: same cost, soaks 1 Damage instead of its Evade, but the Raven costs a Horror, too.
I'm not sure who likes this card. Agnes Baker can maximize it, but she has way better Allies to play....
Stella Clark could benefit from this card. She a low investigate stat and can take 8 horror. Mysterious Raven can also take first 1 horror. And there can be situations where getting clues the normal way is though and then this card gives you 1.