Harry Potter Theory: IT's Pennywise Is A BOGGART!

4. The Shape Of Fear

Boggarts fundamentally seem to embody the human emotion of fear, and consequently feed on other beings' fears. They draw their power from fear, similar to how a Dementor feeds on human despair and sadness. Boggarts too basically eat fear, channelling the most profound fears of its targets specifically in order to increase and feed on that fear.

There are rules though: the boggart isn't just a projection of that fear - a disguise, as it were - the transformation is more complete than that. The boggart literally BECOMES that fear. So if a person is afraid of a werewolf, the boggart will become a werewolf and will be able to do the same sort of damage (albeit on a slightly lessened scale) as a werewolf. If the fear is fire, the boggart will legitimately be able to burn things.

The same goes for It. While the Deadlights have their own power - because they drive anyone who sees them insane, there's nothing to say that that in itself isn't a communal fear passed down and that when anyone sees the Deadlights, all they're seeing is another form of the boggart. After all, two characters in IT and IT Chapter Two are caught in the Deadlights and are somehow capable of escaping. If they're the most fearsome part of IT, they don't live up to their billing.

IT can do the same damage to its victims as the forms of fear it inhabits: as well as more broad forms of attack like violence, Eddie's Leper can pass on infectious diseases proving there is a similar form of specificity capable in IT's "hauntings."

And then there's the point about individual fears again. IT appears to each of the Losers - when they're alone - as their most affecting fears, dropping the Pennywise gimmick for all but Richie Tozier, who expresses a fear of clowns. In reality, his backstory introduced in Chapter Two probably means he's scared of bullies and the need for disguises, but Pennywise fits that best anyway.

IT drives the friends apart because it is more effective when they're separated. That's why it becomes so key that they don't split up and why IT has a greater hold over them when they are. The reason comes back to the boggart mythology. As Wizarding lore confirms, if a boggart appears to multiple victims at once, it can become confused.

In the books, Lupin tells the story about a boggart trying to scare two people at once with different fears. One was afraid of a headless corpse and other was afraid of flesh eating slugs. The resulting half a slug that boggart turned into apparently wasn't frightening. That would account for why IT prefers the kids apart, because even though IT's able to conjure a communal fear, it's not as effective as their own fears.

And as an indication of that, when IT loses some of its power at the end of Chapter Two, we see it cycling uncontrollably through the individual fears, unable to choose the right one. IT might be an incredibly powerful boggart, but once it has had some of that power taken away, it becomes subject to the same limitations.

There's another more specific link that need to be discussed too. Because neither of them can really be killed...