The first benefit of the Sentinel feat says (PHB, p. 169-170):
When you hit a creature with an opportunity attack, the creature's speed becomes 0 for the rest of the turn.
If a character with the Sentinel feat hits a creature, but that creature then gains a new type of speed (e.g. a fly speed), does that new speed also become 0 for the rest of the turn?
For example, say that a spellcaster is fighting a character who has the Sentinel feat.
It is currently the spellcaster's turn. The spellcaster's movement provokes an opportunity attack from the Sentinel character, and it hits, causing the spellcaster's speed to become 0 for the rest of the turn. In turn, the spellcaster then casts the fly spell on themselves, and thus gains a fly speed of 60 feet (a new type of movement for the creature):
You touch a willing creature. The target gains a flying speed of 60 feet for the duration. When the spell ends, the target falls if it is still aloft, unless it can stop the fall.
What is the spellcaster's fly speed (immediately after casting fly)?
Does it also become 0 for the rest of the turn? Or does the spellcaster now have a fly speed of 60 feet that they can use this turn?
From my understanding, there isn't a so called "stack" like there is in Magic: the Gathering. The effect from Sentinel is very specific; it doesn't matter if the creature gains a new type of speed after the Sentinel effect has been applied – all of its speeds are 0 for the rest of the turn. Is my conclusion correct?
Best Answer
Order of operations is what matters here
The Sentinel OA doesn't prevent movement, it just sets the target's speed to 0. Similarly, fly doesn't care how fast you can normally fly or what your land speed is, it just gives you a set flying speed of 60'.
When multiple abilities set the value(s) of a characteristic, the last effect applied is the one that matters. Think of it this way: If you have a carton with 5 eggs and then I take it away and give you new carton with 2 eggs, then take that away and give you a carton with 7 eggs, you have 7 eggs. In fact, it doesn't matter how many eggs you started with, nor how many eggs I gave you the first time: since each carton is a completely different carton and they don't interact in any way, only the one you now have is relevant to figuring out how many eggs you now have.
If, instead of setting your speed to 0, Sentinel used up your movement, however, things would be different. Separate movement modes aren't entirely independent, unlike the effects of the abilities in question. Specifically, when you go to use a new kind of movement during a turn, the following limitation applies:
so if you had used up all your movement for your land speed, as opposed to having it set to 0, then you would only be able to move with your fly speed distance equal to the difference (assuming your fly speed was bigger). That's not what Sentinel actually does, though, so basically the only effect on the caster in your example is that they can't use their walking speed for the rest of their turn.