Sentinel does not prevent a creature from gaining new modes of movement or increasing their movement speed after it has been applied.
I’ll quote the full text of Sentinel, just so we know we aren’t missing anything:
You have mastered techniques to take advantage of every drop in any enemy’s guard, gaining the following benefits:
When you hit a creature with an opportunity attack, its speed becomes 0 for the rest of the turn.
Creatures provoke opportunity attacks from you even if they took the Disengage action before leaving your reach.
When a creature within your 5′ of you makes an attack against a target other than you (and that target doesn’t have this feat), you can use your reaction to make a melee weapon attack against the attacking creature.
First, we have the function in question, the “speed becomes 0” function:
- Whenever you hit a creature with an opportunity attack, its speed drops to 0 for the rest of the turn. This stops any movement they may have been taking.
Lets break it all down.
“When you hit a creature with an opportunity attack,”.
Here we have the triggering condition. This tells us that this only applies on opportunity attacks, which are typically caused by creatures trying to move away from you. So it doesn’t apply on regular attacks from an Attack Action, and it doesn’t apply on Readied Action attacks.
“it’s speed becomes 0 for the rest of the turn.”
Next, we have the effect that is triggered. A creatures speed becomes 0. It doesn’t spend the creatures movement, it doesn’t subtract movement from the creature, it sets it directly to 0, and that setting lasts for the rest of the current turn.
Do you know what is missing from this portion of the ability? It does not have any clause stating that it prevents a creatures movement from being increased again after it has been applied. but maybe there’s something else in the ability that says otherwise. Let’s read on.
The second function:
- Creatures within your reach provoke opportunity attacks even if they took the Disengage action.
This doesn’t really matter much in this situation, it only provides an additional situation where you can spend you reaction to take an opportunity attack, and has no effect that interacts with the speed reduction mechanic.
And the third function:
- When a creature within your reach makes an attack against a target other than you (and that target doesn’t have this feat), you can use your reaction to make a melee weapon attack against the attacking creature.
This also only provides an additional situation in which you can make opportunity attacks: when a creature within your reach attacks someone else who does not have the Sentinel feat. Again, there is not interaction with the speed reduction mechanic.
So where does that leave us? What do we now know?
We know that
- The Sentinel feat sets a creatures speed to 0′ when you hit with an opportunity attack
- The targets current movement ends (and since it has a speed of 0, can’t begin moving again)
- It can make opportunity attacks against a creature even if that creature has taken the Disengage Action
- It can make opportunity attacks if a creature attacks someone who does not have the Sentinel feat.
What do these things tell us? Well, we know that
- The speed change is not a speed reduction. We don’t spend their movement, we don’t subtract anything. The speed is set directly to 0′, do not pass go, do not collect $200.
- There is no clause preventing further speed modifications. If we cast a spell, or activate a special ability, or do some other action that changes our movement speed after Sentinel is applied, the Sentinel feat does not have a clause preventing this.
But how do we know that this is the case?
For comparison, let’s take a look at the Grappled condition:
- A grappled creature’s speed becomes 0, and it can’t benefit from any bonus to its speed.
- The condition ends if the Grappler is incapacitated (see the condition).
- The condition also ends if an Effect removes the grappled creature from the reach of the Grappler or Grappling Effect, such as when a creature is hurled away by the Thunderwave spell.
There is a clause in the Grappled condition that Sentinel does not have “it can’t benefit from any bonus to its speed.” Without this clause, Sentinel does not prevent the target from changing their speed after it is applied.
Some examples of what does and doesn’t work:
These do work, but only if used after getting hit by Sentinel:
The spell Longstrider has the effect “The target’s speed increases by 10 feet until the spell ends.” 0′ + 10′ = 10′, so the target now has a speed of 10′.
The spell Fly has the effect “The target gains a flying speed of 60′ for the duration.” This has two effects. First, it grants the target the ability to fly via a fly speed, second, it sets that speed to 60′. Since it sets it to a specific value, it overrides the Sentinel feat setting it to a specific value.
This doesn’t work:
The Tabaxi’s racial ability Feline Agility, which states “When you move on your tum in combat, you can double your speed until the end of the turn.” Whether it’s used before or after getting hit by Sentinel, it doesn’t help. If it was used before, then we have 60′ speed reduced to 0′ by Sentinel, and if it was used after, then we are doubling the speed we were set to be Sentinel, in other words, 0, because 0 x anything = 0.
And just a reminder for those who may have forgotten: There are no hidden rules in D&D 5e. Abilities spell out exactly how they work. If this ability was intended to prevent a target from further changing their speed, it would say so.
Similarly, there is a difference between can’t move and speed becomes 0, as discussed in this question. Further, in a comparable situation, if a creatures speed becomes 0 due to exhaustion, as asked in this question, nothing prevents it from magically (or otherwise) increasing its speed.