Ray of Frost & Longbow's Slow

Does the -10ft movement for these 2 attacks stack?

Yes.

Multiple instances of the same ability don't stack, but that would be Ray of Frost + Ray of Frost, or Slow + Slow. These are just two abilities with the same effect.

yeah. As long as the SOURCE TY0ES of the same effect are DIFFERENT, they stack.

the effect here is -10ft to movement. the sources are a specific cantrip and a weapon, so they stack.

Eta: if you hit the target with 2 different spells that slow, they stack. 

But i just saw the later comment that points out the specific rule that excludes different weapons. Without that rule, if 2 different longbows hit a target, only 1 would slow. But if a longbow and  whip attacked, then they would stack. But the specific "no weapons with slow ever stack" rule overrides the general.

I assume the weapon rule came about because the party would surround an enemy with 3 types of slow weapons and just keep the enemy pinned to one location.

A good example of this is a level 7+ Eldritch Knight with a Whip and the Slasher feat that uses War Magic to replace an attack with Ray of Frost. You would get 10 feet of slowing from the Slow Mastery on the Whip, another 10 from the Slasher feat for doing slashing damage, and then another 10 from Ray of Frost for a total of 30 feet of slowing. That leaves a whole lot of melee enemies completely unable to move.

 Slow. If you hit a creature with this weapon and deal damage to it, you can reduce its Speed by 10 feet until the start of your next turn. If the creature is hit more than once by weapons that have this property, the Speed reduction doesn’t exceed 10 feet.

Is the above bold then ignored?

Quote from Character77006 >>

 Slow. If you hit a creature with this weapon and deal damage to it, you can reduce its Speed by 10 feet until the start of your next turn. If the creature is hit more than once by weapons that have this property, the Speed reduction doesn’t exceed 10 feet.

Is the above bold then ignored?

 No, it's not being ignored because it doesn't come into effect. That bit in bold only stops you stacking multiple instances of the Slow mastery trait, not multiple instances of speed reduction

Quote from SunIsGettingRealLow96589 >>

yeah. As long as the SOURCE TY0ES of the same effect are DIFFERENT, they stack.

the effect here is -10ft to movement. the sources are a specific cantrip and a weapon, so they stack.

Eta: if you hit the target with 2 different spells that slow, they stack. 

But i just saw the later comment that points out the specific rule that excludes different weapons. Without that rule, if 2 different longbows hit a target, only 1 would slow. But if a longbow and  whip attacked, then they would stack. But the specific "no weapons with slow ever stack" rule overrides the general. 

This isn't quite correct. There is no general rule saying that "if the source types of the same effect are different, they stack" nor is there any general rule saying that the same effect from the same source applied twice doesn't stack.

Multiple instances of the Slow weapon mastery property don't stack because the property's description explicitly says it doesn't. Without that rule, multiple instances of it would stack, regardless of what type of weapon they were applied with.

Multiple instances of Ray of Frost's effect don't stack with each other because there's a general rule that the same spell applied to the same target twice doesn't stack. This rule is specific to spells and does not apply to anything else that's not a spell.

Does that mean a Potion of Speed would stack with the Haste spell? Only one of them is a spell, even though they have (almost) the same effect.

Quote from Frachese >>

Does that mean a Potion of Speed would stack with the Haste spell? Only one of them is a spell, even though they have (almost) the same effect.

Potions that say they duplicate the effect of a specific spell are kind of a weird edge case for this and I don't think there's a clear answer.

To me, it seems like the intent is that the actual Haste spell and a potion that gives you the effect of Haste should not stack, but you can certainly make the argument that the potion isn't actually casting the spell, so that rule doesn't apply.

In that specific case, it's also not super clear how double-stacking Haste would even work, since it has several independent effects and (like a lot of spells) it feels like it was written with the assumption that couldn't be done due to the spell-combining rules.

From a previous discussion regarding stacking and potions:

Quote from Tarod_ >>

Quote from Tana___ >>

Quote from SmiteMakesRight_3_5 >>

Quote from Tana___ >>

And, technically, potions of speed stack with Haste (As they say that you gain the effect of haste, not cast the spell, meaning that it stacks)

That is dubious and I think Combining Spell Effects is clearly intended to apply preventing them from stacking.

However, keep in mind that the effect is triggered off of casting a spell with a spell slot, so that could be a Bonus Action spell, such as Expeditious Retreat. (It could also be to cast Haste or Prismatic Wall in order to get an extra Dash!)

Still, four actions plus a Bonus Action is, pun intended, wild.

 Except your not casting a spell, you're drinking a potion. The spell has nothing to do with it, other than the effects

 There's a SAC answer that addresses the intent:

Do the benefits from Bardic Inspiration and the Guidance spell stack? Can they be applied to the same roll?

Yes, different effects stack if they don’t have the same name. If a creature under the effects of a Guidance spell makes an ability check using the skill chosen as part of the spell’s casting and also has a Bardic Inspiration die, it can roll both 1d4 and 1d6 if it so chooses.

And also this interesting post from Thezzaruz:

Quote from Thezzaruz >>

Quote from Tarod_ >>

 There's a SAC answer that addresses the intent:

Do the benefits from Bardic Inspiration and the Guidance spell stack? Can they be applied to the same roll?

Yes, different effects stack if they don’t have the same name. If a creature under the effects of a Guidance spell makes an ability check using the skill chosen as part of the spell’s casting and also has a Bardic Inspiration die, it can roll both 1d4 and 1d6 if it so chooses.

Yea and then there is this one too:

Do the AC bonuses from a Ring of Protection and Bracers of Defense stack?

Yes. In general, bonuses stack unless the text explicitly says otherwise (such as effects from the same spell). For instance, you can’t benefit from more than one Ring of Protection, because the text says you can’t attune to more than one copy of an item at a time.

 

It seems like kind of a missed opportunity to have two answers about this when both names effects clearly similar but also clearly different. It would have been great if they could have cleared up Potions instead as the language for those is a lot less clear.

Potions generally come in four varieties. 
#1is potions that just gives you an effect, like Potion of Climbing that says "you gain a Climb Speed equal to your Speed for 1 hour" (and some add-ons). Clearly not going to be a problem here.

#2 is Potion of Animal Friendship that says "you can cast the level 3 version of the Animal Friendship spell". A bit weird wording but also clearly affected by the two SAC answers, it won't stack (not that there is anything to stack but still).

#3 is Potion of Heroism that says "you are under the effect of the Bless spell". Also not an issue, clearly a spell effect so it won't stack.

#4 is the majority though and here comes the problem. Spells like Potion of Speed that says "you gain the effect of the Haste spell for 1 minute" (there is a bunch of these that gives out the effect from different spells).

I think that the intention is for these to be viewed as the same as if you had gotten the effect by having the spell cast on you. But I also think that a lot of people views something you get from a potion as different to something you get from a spell (especially as, to me at least, it seems like many of these are written as they are just so that they didn't have to write up a "new" effect from the potion but rather just could re-use an already existing game mechanic).