Ward of Protection is often considered one of the strongest events in the game, since cancelling a critical treachery can be game changing. Deny Existence is a similar card, that has narrower coverage, but does a better job for what it covers. Compared to Ward of Protection, this card cost 1 resource and 1 horror less to play, and more importantly, it can be played after you failed a test on a treachery.
Imagine you are Agnes Baker and has 3 sanity remaining. You draw Rotting Remains. Normally you would be happy since you can usually pass the test with ease, but since an auto-fail draw outright defeats you, you may consider using Ward of Protection to cancel it. However, if you have Deny Existence in your hand, you may instead take the chance. Most of the time you will pass the test, and if you auto-failed you just play the card to ignore it. In this aspect, Deny Existence to Ward of Protection is like what Lucky! is to Unexpected Courage, that it has an effect even if you do not play it. But the cost is also favorable to Deny Existence. This shows how insanely powerful is the card.
Additionally, Deny Existence can cancel certain weaknesses, since treachery weaknesses are considered encounter cards. Notably it ignores Amnesia and Paranoia in their entirety. This is another really strong ability. Interesting side note: The card can ignore the direct horror from To Fight the Black Wind, and potentially preventing the doom as well.
Being able to ignore enemy attacks is the icing on the cake. Most enemies deal both damage and horror though, so Deny Existence usually only ignore part of the attack.
Deny Existence is such an excellent card, that it is hard to imagine not putting it in every deck that can take it.