- Q: For cards like Deny Existence, "I've had worse…", Delay the Inevitable, etc, do they directly or indirectly work against effects that do not deal damage and horror directly to your investigator (e.g. Snake Bite) or deal direct damage to other things as well as your investigator (e.g. Centuries of Secrets, Ceremony Room)? A: Delay the Inevitable, I've Had Worse and Deny Existence only work when you (aka your investigator) are being dealt damage/horror. If damage or horror is being dealt directly to a card other than your investigator, you cannot use Deny Existence, Delay the Inevitable, or I've Had Worse to cancel/ignore it.
Sort. Paradoxe.
Rapide. À jouer quand une carte Rencontre ou une attaque ennemie devrait vous infliger l'une des choses suivantes (choisir une option) : défausser des cartes de votre main, perdre des ressources, perdre des actions, subir des dégâts ou subir des horreurs.
Ignorez cet aspect de l'effet. (Chaque autre aspect se résout normalement.)

- Deny Existence (5) In the Clutches of Chaos #280
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.
Not to mention the spell synergy (Arcane Initiate), ignore synergy (Diana Stanley) and event synergy (Sefina Rousseau) the card provides.
Deny Existence is such an excellent card, that it is hard to imagine not putting it in every deck that can take it.
The FAQ from this page is extremely relevant for this card:
"The act, agenda, locations, scenario reference card, locations, and all cards in the encounter deck are all considered encounter cards."
So you can use Deny Existence on all of those cards. A location makes you take damage or lose actions when you enter it? Ignore it! The Agenda flips and deals you a bunch of horror? Not a problem!
Spoilers for Innsmouth: You can even use this card to stop yourself from drowning. Why not take Dianna to The Pit of Despair and spend the whole scenario underwater?
This card is best with Diana, for whom (as long as you have an open spot beneath her) it replaces itself and gives +1 resource (same net bonus as Emergency Cache). It might be usable in some other decks, especially if you are trying to increase your spell count (for Arcane Initiate, etc). Awesome artwork, too!