Can a player begin and end its movement inside a swarm's space and not incur an opportunity attack?

Question

Dirk, Hermes, Necryx, Jomil and Sing Lore- avert your eyes, or incur the wrath of Spuggroth the Humbled and Aflame!

I'm designing an encounter where the party is trying to confront some foes in the midst of a massive bar fight, which I'm thinking of treating as two gargantuan, overlapping swarms fighting each other (the party has a stake in which side wins the bar fight, and the encounter will grow more dangerous if their side starts losing)

For reference, the swarm ability includes the following text:

Swarm. The swarm can occupy another creature's space and vice versa

It's established that moving out of the 0 ft. reach of a swarm triggers an opportunity attack as per usual, but if I create a gargantuan swarm, it's very likely a player could begin and end it's movement while remaining entirely within the swarm.

Using the rules as written, I'd assume a character moving in this way would not trigger an opportunity attack, but it does seem a bit counterintuitive. I can't think of any gargantuan swarms in published 5e content, and this might be why, but if anyone has some insights on how to rule this, or if there was a clarification published somewhere, I'd love to hear it.

Answer 1

Staying within a swarm's space means no opportunity attacks are provoked

The Swarm trait states:

The swarm can occupy another creature's space and vice versa [...]

Meanwhile, opportunity attacks are only provoked when:

[...] a hostile creature that you can see moves out of your reach. [...]

You have to actually move out of their reach, so moving through a creature's space, by any means, does not provoke an opportunity attack. This is also the case with a Large or larger swarm.

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Just an aside to point out that there are, at least currently, two Large or larger swarms (both are Large): the Skeletal Swarm and the Sahuagin Hatchling Swarm, both from Ghosts of Saltmarsh. They have the Swarm trait, and I would assume that a creature moving from one part of their space to another part of their space would not provoke an opportunity attack.

Answer 2

Homebrew creatures require homebrew adjudications

A swarm is typically a Medium creature "made" from very small low-Intelligence creatures like insects, which moves and acts as a whole. There really isn't an equivalent official creature to base a ruling off of. Official source books have no Gargantuan swarms consisting of Medium/Small creatures.

Gargantuan swarms are not the recommended solution for resolving mass combat. This approach will probably have various problems aside from opportunity attacks. For instance, a spell which targets a creature now targets the whole crowd.

I'd recommend using the "Mass Combat" UA or just leaving the bar brawl as a narrative background for the main encounter.

Speaking of the initial question, you've already answered it:

Using the rules as written, I'd assume a character moving in this way would not trigger an opportunity attack, but it does seem a bit counterintuitive.

So yes, moving inside a creature's space does not trigger OAs, and yes, it is counterintuitive and probably unbalanced. However, as an author you can "fix" it by explicitly describing a trait that works the way you want it to. Something like this:

Angry Mob. When a hostile creature moves inside the crowd for at least 10 feet, the crowd can use its reaction to make an opportunity attack against that creature.