The Decanter of Endless Water, DMG pg. 161, can fire a 30 foot long, 1 foot wide stream of water that can knock a target prone. On top of that, it keeps streaming until the end of my next turn (important).

A Vampire, MM pg. 297, takes 20 acid damage if it ends its turn in running water.

If I somehow manage to get this going on a vampire that we've prevented from moving, would this count as running water?


What Jeremy Crawford Says

Jeremy Crawford answered the question of what is running water. To the question:

... Is this only from natural running water, or any water that move?

Jeremy answered:

A body of running water from any source can harm a vampire.

He then clarified, when asked about water elementals:

A water elemental is a creature, not a body of running water (a stream, a river, a waterfall, or the like).


How it applies

So, based on that, If you can get enough water out of the decanter that it is stream or river like for your DM, then you're good. But even with the clarifications it is a DM call. I personally think 1 foot x 1 foot of moving water is enough to count as a body of running water.

I, personally, would give the vamp a dex save (DC 15??) to see if he can dodge it before it hits him.

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    \$\begingroup\$ They already get a Strength save against it knocking them over. \$\endgroup\$ – Lino Frank Ciaralli Mar 15 '17 at 17:34
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    \$\begingroup\$ He specifically said "body of water." This generally implies a much larger thing than a 1' x 1' thing. \$\endgroup\$ – BobTheAverage Mar 16 '17 at 1:10
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    \$\begingroup\$ @MarcusYoder Don't forget it's 1' x 1' x 30'. I'd say a natural stream 1' wide and 1' deep would certainly count. \$\endgroup\$ – Angew is no longer proud of SO Mar 16 '17 at 7:46
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    \$\begingroup\$ He also said "from any source," and a DoEW is a source. Which is why I've accepted this answer. \$\endgroup\$ – Lino Frank Ciaralli Mar 17 '17 at 12:32
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    \$\begingroup\$ The stream may be 1 foot wide, but that doesn't make it a solid sheet of water. A geyser of water is broken up into many fast moving droplets. This is very different than a slow moving solid stream. \$\endgroup\$ – BobTheAverage Mar 17 '17 at 16:56

I think you've exhausted the specific rules in your question, so you're going to need your DM to rule on this, or if you're a DM - decide!

  • I would certainly reward player ingenuity and allow this effect for one turn.
  • But it would also be possible to make a case for the Vampire not being "in" running water (in the same way it would be if it were in a river). Also the DM could rule that the water canon knocks it back, and out of the way of the water.

In general, damaging effects require an attack roll or saving throw. Since none are given in the item description, your DM may assign one or rule that this is not a damaging effect. Another important question is whether your character knows about running water and vampires.

  • \$\begingroup\$ I'm not sure why so much dogpiling on this answer. Can the next person to downvote please explain why in a comment. \$\endgroup\$ – Clearly Toughpick Mar 16 '17 at 9:12
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    \$\begingroup\$ Haven't downvoted, but this answer is inaccurate as not all damaging effects require an attack roll or saving throw. As stated in the question: "A Vampire, MM pg. 297, takes 20 acid damage if it ends its turn in running water." \$\endgroup\$ – Halaster Mar 16 '17 at 9:54
  • \$\begingroup\$ The answer does not say all damaging effects. It says " in general". The OP already quoted rule from MM about vampires. The question is about using the decanter as a weapon. Interestingly, the most highly voted answer suggests a DC 15 DEX save. @LucasCordina \$\endgroup\$ – Solanacea Mar 16 '17 at 13:15
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    \$\begingroup\$ In general, players should be strongly encouraged to find bizarre ways to solve problems and destroy monsters. \$\endgroup\$ – rosuav Mar 16 '17 at 13:57
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    \$\begingroup\$ I am gonna agree with @Solanacea 's comment. The as written rules are ambiguous, so the DM should make a ruling consistent with the imagined universe and consistent with other rules. \$\endgroup\$ – BobTheAverage Mar 17 '17 at 16:55

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