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The spirit guardians spell has the effect of halving a target's speed if they are in the area of effect. I don't quite follow how to apply this though. If it were the start of their turn I could understand - I'd just halve their speed.

But what about if they move into the area of effect? My understanding is that a character gets their current speed score in movement at the start of their turn (barring other effects). Is that altered if their speed drops by half (like losing maximum HP)? Or is it only a problem at the start of their turn?

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How you treat it depends on exactly where the character starts

Spirit Guardians interacts with the rules in an odd manner with respect to speed, going by RAW

Speed is defined as:

Every character and monster has a speed, which is the distance in feet that the character or monster can walk in 1 round. This number assumes short bursts of energetic movement in the midst of a life-threatening situation.

In the case of spirit guardians the effect states (emphasis mine):

An affected creature's speed is halved in the area, and when the creature enters the area for the first time on a turn or starts its turn there, it must make a Wisdom saving throw.

So while the character is in the area their speed is halved. So if a character normally has a speed of 30ft, and they start their move in the area, they have a speed of 15ft while in the area. The same is true of a creature that starts its turn outside the area, but moves into the area. While in the area their speed is halved.

Take this example:

  • Character A: 30ft speed normally, starts turn 30ft away from the centre of the spell
  • Character B: 30ft speed normally, starts turn inside the area, 5ft from the edge
  • Character C: 30ft speed normally, starts turn in the centre of the area

Character A can spend 15ft of movement to get to the edge of the spell area, but as soon as they enter the area their speed is halved. By entering the area they will have spent 20ft of movement, but their speed is 15ft. As a result they can no longer move.

Character B, starts off with 15ft speed due to the spell. They can spend 5ft of movement to exit the area. Since they are no longer in the area their speed is no longer halved. As a result their speed is now 30ft again, they have only spent 5ft of it, so they have 25ft left to spend as they please.

Character C can spend 15ft of movement to get to the edge of the spell effect, but cannot leave it. As a result they have a speed of 15ft for the entirety of their turn and their movement ends when they get to the edge of the spell.

Some of this may seem a little absurd, but it's how the rules read.

Alternate solution, treat the spell's area as difficult terrain for a more consistent application

A more manageable way to treat this spell, as a DM, would be to rule the area in the spell counts as difficult terrain. In difficult terrain a creature must spend 10ft of their speed to move 5ft. This would make the time spent in the area consistently effect all characters, regardless of where in they start in respect to the centre of the spell.

For situations where the spell is overlapping with already difficult terrain, in order not to reduce the spells effectiveness, make sure to double the difficulty of the terrain within the spells area. So when the spell's area is on top of difficult terrain a creature would need to spend 4ft of speed to move 1ft. Rangers and Druids should also not likely be immune to this change from halving speed to difficult terrain. (Thanks to @PixelMaster and @MarkWells for pointing these two issues with the ruling out)

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    \$\begingroup\$ make sure to remember that the "difficult terrain" would have to stack with other difficult terrain (as in: DT + spell = 4 ft per 1 ft movement) - otherwise, you restrict the power of the spell. \$\endgroup\$ – PixelMaster Dec 26 '18 at 1:54
  • \$\begingroup\$ @PixelMaster fair point. \$\endgroup\$ – illustro Dec 26 '18 at 1:58
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    \$\begingroup\$ Also, rangers and druids probably shouldn't be immune to the effect (it's not the terrain as such). It's more like the additional movement cost of climbing or swimming. \$\endgroup\$ – Mark Wells Dec 26 '18 at 2:03
  • \$\begingroup\$ @MarkWells noted \$\endgroup\$ – illustro Dec 26 '18 at 2:08
  • \$\begingroup\$ @markswells rangers only ignores non-magical difficult terrain, so is don't think that would matter. \$\endgroup\$ – Garret Gang Dec 27 '18 at 17:18

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