Take a bag of holding, then cast gate on the inside, connect the gate to the infinite sea of holy water around Mount Celestia. Tie the bag shut, and then open it in the face of fiends or undead. KABOOM!
If you overload a bag of holding, it ruptures and its contents spill into the Astral Plane.
Deities and planar rules can prevent gates from opening within their domains.
The inside of the bag is an extradimensional space, which means it's on another plane. It's arguably not possible to cast Gate targeting the inside of the bag from outside of the bag, as two planes don't really have a defined distance between them.
The D&D rules generally don't treat non-solids as objects, and water tends to be treated as just another form of atmosphere. As a DM, I wouldn't allow a Gate opened from the Plane of Water to flood the target plane any more than I'd allow a Gate from the Material Plane to the void of space to drain the planet's atmosphere.
Didn't something similar to this happen on Critical Role when Orion Acaba tried to get his decanter of endless water blessed to become unlimited holy water? I think the final decision is that only the water inside the decanter at the time of the blessing became holy water.
The volume within the bag of holding is 64 cubic feet. This means any given dimension of height, width or depth is only 4 feet.
That's not a truism (discussed here). But even if that were the case, it could be circumvented by using a Portable Hole instead.
The problems with Portable Hole is that the space within is a different plane (as stated in the item description) and so you would be unable to target it for placement of Gate. Even if this was handwaived and you could successfull fill the hole with holy water the utility of this is extremely limited. To open the hole it must be unfolded against a solid surface. If you place it on the ground it's just a hole filled with holy water, if you place it on a wall the holy water just spills out and is unlikely to affect anything and so so you can either place on the ground and trick the fiend into stepping into it or somehow place it on the ceiling directly above them. Both are possible but difficult. Quite frankly it would be much easier to just buy or make the holy water.
If you want a trap that definitely works with RAW: combine Demiplane with Gate.
1. create the opening for demiplane on ground, create gate next to it so water spills out into the demiplane doorway. You'll have 30 foot cube demiplane (27,000 cubic feet) of holy water. Later when in battle just use Demiplane to create a doorway above the fiend and they get a waterfall of holy water on them - enough volume of it to pretty much insta-kill any fiend or undead.
1. Create the opening for Demiplane on the ground.
2. Create the Gate right next to the Demiplane's doorway so the water rushes out and into the doorway filling the demiplane. If you want to fill it completely it might take 2 or 3 days, but hey, it's worth it for insta-killing almost any fiend or undead. Demiplane is 30 feet cube which means 27,000 cubic feet in volume. A small flash of holy water does 2d6 radiant and you have holy water to fill two swimming pools.
3. WHen you encounter the fiends or undead just use Demiplane to open a door above them. Out comes a waterfall of rushing holy water. It's enough to affect several targets at once and can deal enough radiant damage to instantly kill most fiends and undead. It is also rushing water making it especially effective against vampires.
Alternatively,
If you had time to prepare try combining Demiplane with Wish replicating Glyph of Warding housing a spell like Sunburst. Every day you have to prepare is another glyph and glyphs can be small so you could have a lot. When you have it ready and face fiends and undead you can open the demiplane below them or force them through the doorway. As soon as they pass through the doorway all those glyphs with sunburst go off dealing 12d6 radiant damage each.
You could be better off using Disintegrate rather than a Radiant damage spell as it does more damage and no monster is immune so with enough glyphs even if they have resistance and save all of them it can still do enough damage to instant kill. This is especially brutal if you polymorh the creature first (at 0 hp they don't just turn back, they disintegrate and die) because it would take only 1 glyph to instant-kill them.
Alternatively,
Combine Demiplane with Forbiddance.
1. Make a Demiplane and put an object with a made up word inside it. This ensure the Demiplane is created and stays permanently.
2. Cast forbiddance to cover the area but only up to 29 feet high. If you cast this in here everyday for 30 days it becomes permanent. If have Wish you can use that even if not a cleric and don't have to spend on component costs.
3. When facing enemies open a doorway targeting the ceiling of your demiplane. You can create doorway to any surface of your demiplane, after all.
4. Enjoy. Non-Flying creatures cannot get out and if they can fly a dispel magic on the doorway can close it early. Forbiddance prevents any portals or planar travel - your portal into it was on the ceiling, 1 foot away from where forbiddance begins so the doorway is not affected but the space between doorway and forbiddance is too small to use as a safe zone. Since it blocks planar travel they cannot teleport/gate/plane shift out of it. If the doorway is closed then they are well and truly trapped. Taking 5d10 radiant damage when it enters and on every turn forever. By the next day anything inside would be dead and since the demiplane and forbiddance are permanent you can use this again and again. Since it's level 8 you could use this trap twice a day. You now have a permanent guanatee-kill box for any creature not immune to radiant damage that you can get through the doorway. There is also nothing stopping you creating another one for necrotic damage because there is no limit to the number of permanent demiplanes you can create or permanent zones of forbiddance.
If you're going for auto-kills there are far better and perfectly in-RAW combinations you can do that are far better than combining a bag or hole with Gate for holy water.
If you want a trap that definitely works with RAW: combine Demiplane with Gate.
1. create the opening for demiplane on ground, create gate next to it so water spills out into the demiplane doorway. You'll have 30 foot cube demiplane (27,000 cubic feet) of holy water. Later when in battle just use Demiplane to create a doorway above the fiend and they get a waterfall of holy water on them - enough volume of it to pretty much insta-kill any fiend or undead.
1. Create the opening for Demiplane on the ground.
2. Create the Gate right next to the Demiplane's doorway so the water rushes out and into the doorway filling the demiplane. If you want to fill it completely it might take 2 or 3 days, but hey, it's worth it for insta-killing almost any fiend or undead. Demiplane is 30 feet cube which means 27,000 cubic feet in volume. A small flash of holy water does 2d6 radiant and you have holy water to fill two swimming pools.
3. WHen you encounter the fiends or undead just use Demiplane to open a door above them. Out comes a waterfall of rushing holy water. It's enough to affect several targets at once and can deal enough radiant damage to instantly kill most fiends and undead. It is also rushing water making it especially effective against vampires.
Alternatively,
If you had time to prepare try combining Demiplane with Wish replicating Glyph of Warding housing a spell like Sunburst. Every day you have to prepare is another glyph and glyphs can be small so you could have a lot. When you have it ready and face fiends and undead you can open the demiplane below them or force them through the doorway. As soon as they pass through the doorway all those glyphs with sunburst go off dealing 12d6 radiant damage each.
You could be better off using Disintegrate rather than a Radiant damage spell as it does more damage and no monster is immune so with enough glyphs even if they have resistance and save all of them it can still do enough damage to instant kill. This is especially brutal if you polymorh the creature first (at 0 hp they don't just turn back, they disintegrate and die) because it would take only 1 glyph to instant-kill them.
Alternatively,
Combine Demiplane with Forbiddance.
1. Make a Demiplane and put an object with a made up word inside it. This ensure the Demiplane is created and stays permanently.
2. Cast forbiddance to cover the area but only up to 29 feet high. If you cast this in here everyday for 30 days it becomes permanent. If have Wish you can use that even if not a cleric and don't have to spend on component costs.
3. When facing enemies open a doorway targeting the ceiling of your demiplane. You can create doorway to any surface of your demiplane, after all.
4. Enjoy. Non-Flying creatures cannot get out and if they can fly a dispel magic on the doorway can close it early. Forbiddance prevents any portals or planar travel - your portal into it was on the ceiling, 1 foot away from where forbiddance begins so the doorway is not affected but the space between doorway and forbiddance is too small to use as a safe zone. Since it blocks planar travel they cannot teleport/gate/plane shift out of it. If the doorway is closed then they are well and truly trapped. Taking 5d10 radiant damage when it enters and on every turn forever. By the next day anything inside would be dead and since the demiplane and forbiddance are permanent you can use this again and again. Since it's level 8 you could use this trap twice a day. You now have a permanent guanatee-kill box for any creature not immune to radiant damage that you can get through the doorway. There is also nothing stopping you creating another one for necrotic damage because there is no limit to the number of permanent demiplanes you can create or permanent zones of forbiddance.
If you're going for auto-kills there are far better and perfectly in-RAW combinations you can do that are far better than combining a bag or hole with Gate for holy water.
You could also kill every fiend in the lower planes by using the gate in the bottom of the River Styx, which goes through all the planes (lower), and thus it floods them all with holy water. I have redeemed myself from the bag of holding idea!
1. Make a Demiplane and put an object with a made up word inside it. This ensure the Demiplane is created and stays permanently.
2. Cast forbiddance to cover the area but only up to 29 feet high. If you cast this in here everyday for 30 days it becomes permanent. If have Wish you can use that even if not a cleric and don't have to spend on component costs.
3. When facing enemies open a doorway targeting the ceiling of your demiplane. You can create doorway to any surface of your demiplane, after all.
4. Enjoy. Non-Flying creatures cannot get out and if they can fly a dispel magic on the doorway can close it early. Forbiddance prevents any portals or planar travel - your portal into it was on the ceiling, 1 foot away from where forbiddance begins so the doorway is not affected but the space between doorway and forbiddance is too small to use as a safe zone. Since it blocks planar travel they cannot teleport/gate/plane shift out of it. If the doorway is closed then they are well and truly trapped. Taking 5d10 radiant damage when it enters and on every turn forever. By the next day anything inside would be dead and since the demiplane and forbiddance are permanent you can use this again and again. Since it's level 8 you could use this trap twice a day. You now have a permanent guanatee-kill box for any creature not immune to radiant damage that you can get through the doorway. There is also nothing stopping you creating another one for necrotic damage because there is no limit to the number of permanent demiplanes you can create or permanent zones of forbiddance.
If you're going for auto-kills there are far better and perfectly in-RAW combinations you can do that are far better than combining a bag or hole with Gate for holy water.
RAW, I'm not sure that you can cast Forbiddance in a smaller radius than the spell states in its description. Note that it says " to a height of 30 feet above the floor" rather than "up to a height of 30 feet above the floor."
1. Make a Demiplane and put an object with a made up word inside it. This ensure the Demiplane is created and stays permanently.
2. Cast forbiddance to cover the area but only up to 29 feet high. If you cast this in here everyday for 30 days it becomes permanent. If have Wish you can use that even if not a cleric and don't have to spend on component costs.
3. When facing enemies open a doorway targeting the ceiling of your demiplane. You can create doorway to any surface of your demiplane, after all.
4. Enjoy. Non-Flying creatures cannot get out and if they can fly a dispel magic on the doorway can close it early. Forbiddance prevents any portals or planar travel - your portal into it was on the ceiling, 1 foot away from where forbiddance begins so the doorway is not affected but the space between doorway and forbiddance is too small to use as a safe zone. Since it blocks planar travel they cannot teleport/gate/plane shift out of it. If the doorway is closed then they are well and truly trapped. Taking 5d10 radiant damage when it enters and on every turn forever. By the next day anything inside would be dead and since the demiplane and forbiddance are permanent you can use this again and again. Since it's level 8 you could use this trap twice a day. You now have a permanent guanatee-kill box for any creature not immune to radiant damage that you can get through the doorway. There is also nothing stopping you creating another one for necrotic damage because there is no limit to the number of permanent demiplanes you can create or permanent zones of forbiddance.
If you're going for auto-kills there are far better and perfectly in-RAW combinations you can do that are far better than combining a bag or hole with Gate for holy water.
RAW, I'm not sure that you can cast Forbiddance in a smaller radius than the spell states in its description. Note that it says " to a height of 30 feet above the floor" rather than "up to a height of 30 feet above the floor."
But that's the point, isn't it? Sure, a DM may say something like that, but it doesn't work, RAW, which means it's not always reliable. And you can't really blame a DM for saying "no, you have to do it within the measurements," especially when the reason for doing this is so that you can cheese your way through any fiend encounter ever.
3. WHen you encounter the fiends or undead just use Demiplane to open a door above them. Out comes a waterfall of rushing holy water.
Creating the door requires a flat ceiling, and then someone still has to open the door.
When you have it ready and face fiends and undead you can open the demiplane below them or force them through the doorway. As soon as they pass through the doorway all those glyphs with sunburst go off dealing 12d6 radiant damage each.
You still have the problem of opening of opening a door a fiend is currently standing on with this plan.
You could be better off using Disintegrate rather than a Radiant damage spell as it does more damage and no monster is immune so with enough glyphs even if they have resistance and save all of them it can still do enough damage to instant kill.
Not that it matters much but Helmed Horrors are immune.
This is especially brutal if you polymorh the creature first (at 0 hp they don't just turn back, they disintegrate and die) because it would take only 1 glyph to instant-kill them.
Keep in mind the interaction between Polymorph and at-0-HP rules is RAW but goes against RAI (and in fact Mass Polymorph uses different rules based on temporary HP), so your mileage may vary per DM.
Combine Demiplane with Forbiddance.
1. Make a Demiplane and put an object with a made up word inside it. This ensure the Demiplane is created and stays permanently.
2. Cast forbiddance to cover the area but only up to 29 feet high. If you cast this in here everyday for 30 days it becomes permanent. If have Wish you can use that even if not a cleric and don't have to spend on component costs.
3. When facing enemies open a doorway targeting the ceiling of your demiplane. You can create doorway to any surface of your demiplane, after all.
4. Enjoy.
Forbiddance doesn't give you control over the height of the area, though the door would arguably be placed where the spell's area ends anyways. The bigger problem is that Demiplane doesn't give you control over where the door appears on the demiplane side. The DM could decide it's always at floor level within the demiplane room. Another way to run it is that it its location is based on the orientation of the door in your original plane, but that results in ambiguities if you create a door on a slope.
Take a bag of holding, then cast gate on the inside, connect the gate to the infinite sea of holy water around Mount Celestia. Tie the bag shut, and then open it in the face of fiends or undead. KABOOM!
Won't work.
The volume within the bag of holding is 64 cubic feet. This means any given dimension of height, width or depth is only 4 feet.
4 ft high x 4 ft wide x 4ft depth/length = 64 cubic feet.
The minimum dimensions of Gate is a circle of 5 feet. It will not fit inside the storage area of the Bag of Holding.
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There's additional issues here:
Didn't something similar to this happen on Critical Role when Orion Acaba tried to get his decanter of endless water blessed to become unlimited holy water? I think the final decision is that only the water inside the decanter at the time of the blessing became holy water.
"Not all those who wander are lost"
Yeah, I have not seen critical role, but that was my first idea.
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This is lawful evil at its best.
You could also kill every fiend in the lower planes by using the gate in the bottom of the River Styx, which goes through all the planes (lower), and thus it floods them all with holy water. I have redeemed myself from the bag of holding idea!
And that's all I have to say about that.
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And that's all I have to say about that.