
One of the "take a bunch of damage and.or horror" signature weaknesses. Looking at the two elements, the effect and the discard condition, we get:
The effect: You take 0-4 horror, which is unlikely to kill Mark, but it can lose a scenario or campaign and getting Mental Trauma just increases the deadliness of the card. Fortunately for Mark, Guardians have gotten better healing options since Carcosa, and tools like Hallowed Mirror, Second Wind, and even Thermos can help keep Mark's damage low, and there are plenty of Guardian Allies with 2-3 Horror soak to keep Mark on an even keel. If you don't go wild with Sophie before this Weakness drops, Mark will probably be OK. Good thing it doesn't say "direct horror." As a side note, something in the wording of the card rubs me the wrong way, but its meaning is clear.
The discard condition: You draw it, it's Revelation effect happens, you discard it. Simple, but it can happen multiple times, if you deck yourself.
All in all, this is a below average signature weakness. It can ruin your day at the wrong moment, but if you build and play with it in mind, it is unlikely to have a severe impact.