The rules say a Tiny creature can move into or through a Medium creature’s space. Can it end its movement in that space? Can the Medium creature do anything about it?
A Tiny creature can indeed move into or through a square another creature occupies, friend or foe. The other creature doesn’t have to be Medium size — a Tiny, Diminutive, or Fine creature can enter any occupied space. When the little creature enters a space that a foe of Medium size occupies, the gets an attack of opportunity when the Tiny creature leaves the adjacent square to enter the Medium creature’s space, thanks to its 5-foot reach. [More…]
The rules don’t actually come out and say that a Tiny creature can end its movement in another creature’s space (though you can argue about what “into or through” means in this context). If you think about it, however, they must be able to stop in occupied squares or they’d never be able to use their 0 reach to make a melee attack (sans a feat that might allow them to fight on the move).
Okay, what about creatures of different sizes? If you’re three sizes bigger, or three sizes smaller than another creature, you can move through its space. Can you also stop in that space?
You cannot stop in another creature’s space unless that creature is helpless—or if you’re size Tiny, Diminutive, or Fine.
So, Tiny and smaller creatures have 0 reach. How does that affect attacks of opportunity or flanking? For example, how would this situation play out?
A Tiny viper moves into a rogue’s square, provoking an attack of opportunity. The viper survives and attacks.
The rogue uses a move action to leave the space with the viper and moves his speed. Does the viper get an attack of opportunity when the rogue leaves? If yes, why does a swarm of Tiny creatures not make an attack of opportunity?
Yes, the viper makes an attack of opportunity when the rogue leaves the space. You threaten any space where you can make a melee attack and you make an attack of opportunity when a foe leaves a space you threaten.
Creatures with reach 0 cannot flank other creatures.
Swarms do not make melee attacks, so they don’t make attacks of opportunity, no matter what size the individual creatures in them are. The rules don’t explain why that is.
In game design terms, the swarm’s ability to hit automatically at the end their move and all a swarm’s special qualities replace the ability to threaten a space. In terms of what actually going on in the game world, there are any number of reasons why a swarm doesn’t make opportunity attacks. Perhaps creatures in a swarm are too busy crawling all over each other to threaten a space.
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