- Answering the question "What happens if two investigators each have The Thing That Follows as their random basic weakness (as they have constructed their decks from separate collections) and the second The Thing That Follows is drawn while the first is in play?": "Good question! The first Thing stays in play and the second Thing is discarded without effect."
Monster. Curse.
Spawn - Location farthest from you.
Prey - Bearer only.
Hunter.
Forced - When The Thing That Follows would be defeated: Instead shuffle it into its bearer's deck.

This is my favorite Basic Weakness, and I'm writing this review in the hopes that future game designers can take a hint on how to do mechanics like this well.
First of all, while by far my favorite, it is not the card I would chose to put in a deck (not that any of us would pick and choose with a random card selection, right? Riiiiiight?). But when I draw any other random weakness, my thoughts tend to range from "okay, I can deal with this," to "aw, crap. This is gonna be a pain to deal with." The Thing That Follows is the only Basic Weakness that instills an actual emotion in me- a sense of giddy dread.
This card is like a well paced horror movie. It makes every draw an anxious roll of the dice, wondering if this time the unseen Thing is going to crawl out and mess up your day. And while that can be said of pretty much any weakness card, the trick of The Thing That Follows is how it repeats itself. Once you draw the card, the cycle of tension repeats as it moves closer and closer to you across the map. Even if you manage to beat it into submission (not all that unlikely with its stats, but hard enough to mess up some endgame plans), it just returns to your deck, only this time with significantly fewer cards on hand to lower the chances of picking it up again.
It is a masterpiece of the tension/release cycle that is crucial to all forms of horror. Long buildups of dread as you feel it coming, punchy moments of action as it resolves, and repeat. Not scary because it ruins your day, but because no mater how many ways you find to deal with it, it always Follows.
Helping matters is that it's one of the most narratively cohesive Basic Weaknesses out right now. (What, exactly, does Paranoia have to do with resources?) But with the Thing, everything from the name to the art to the quote to the mechanics all serve to instill that key emotion. Not fright, but dread. Consuming, unavoidable dread.
Bravo, Fantasy Flight. You've made a business out of turning Cosmic Horror into game mechanics, and with this one you knocked it out of the park.
Recursive weaknesses require much more management than others, unless you are playing a slow draw deck or maybe Tommy. While this may not be a particularly tough monster, drawing it 4-5 times in a scenario can really slow your tempo.
You'll either need to invest in some cards that let you discard from your deck, like Alyssa Graham or Scroll of Secrets, or keep the Thing alive and evaded until your deck flips. Bind Monster can be useful, or Banish if the map is large enough. Survivors might try traps or tricks, like Hiding Spot or Snare Trap to keep it neutralized while they draw through the rest of their deck.
Finally, Mind Wipe is the ultimate card to defeat this enemy for good, as it would clear the forced effect and let the monster discard as normal. Hope you have someone who can play Mystic 1 events!
Relevant "prey" + "only" rule for reference :
If an enemy's prey instructions contain the word "only," that enemy only moves towards and engages that investigator (as if it were the only investigator in play), and ignores all other investigators while moving and engaging. Other investigators may use the engage action or card abilities to engage the enemy.
- Non-bearer cannot stand on its way to get the engagement.
It is also costly to help because you need an engage action. - If the bearer got eliminated/resigned from the campaign it simply stay still and everyone else can pass through.