Help me understand spell areas please

[Jon_Dahl]

I'm designing a trap for my players. A Black Tentacle Trap.

The area is a simple wooden house. Downstairs, there's only one room, 30*30 ft. There are stairs to upstairs (5 ft wide, 15 ft long). Stairs are by the wall. The door in upstairs in locked.

When players climb up stairs, the alarm (which is the spell trigger) is guarding one of the steps. It's in the half-way of the stairs, so the trigger is basically in the 0.5th floor.

Black Tentacles has 20 ft spread. So when the spell spreads from that certain step halfway of the stairs, it climbs up the stairs and stops in front of the door. Then it descends and fills the room below with black tentacles, but not fully because of spread is not long enough to reach wall to wall. The stairs will be entirely covered with black tentacles. However if there's an empty space under the stairs, it will not be filled with them, because the stairs are already filled with tentacles and they, you know, they can't... well they can't double like that!

In total there will 8-11 tentacles, which must be divided equally among the squares, keeping sure that no more than six tentacles are in adjacent squares. Considering the space, this is not a problem.

Did I get this right? I hope you understood my explanation! Ask, and I will explain it better!

Edit: I'm playing 3.5

 

[Empirate]

The answer to your question depends on which edition you're playing: your reference to a number of tentacles seems to imply you're playing 3.0. That I can't help you with, unfortunately. I can, however, tell you how it works in 3.5:

Area

Some spells affect an area. Sometimes a spell description specifies a specially defined area, but usually an area falls into one of the categories defined below.
Regardless of the shape of the area, you select the point where the spell originates, but otherwise you don’t control which creatures or objects the spell affects. The point of origin of a spell is always a grid intersection. When determining whether a given creature is within the area of a spell, count out the distance from the point of origin in squares just as you do when moving a character or when determining the range for a ranged attack. The only difference is that instead of counting from the center of one square to the center of the next, you count from intersection to intersection.
You can count diagonally across a square, but remember that every second diagonal counts as 2 squares of distance. If the far edge of a square is within the spell’s area, anything within that square is within the spell’s area. If the spell’s area only touches the near edge of a square, however, anything within that square is unaffected by the spell.
Burst, Emanation, or Spread

Most spells that affect an area function as a burst, an emanation, or a spread. In each case, you select the spell’s point of origin and measure its effect from that point.
A burst spell affects whatever it catches in its area, even including creatures that you can’t see. It can’t affect creatures with total cover from its point of origin (in other words, its effects don’t extend around corners). The default shape for a burst effect is a sphere, but some burst spells are specifically described as cone-shaped. A burst’s area defines how far from the point of origin the spell’s effect extends.
An emanation spell functions like a burst spell, except that the effect continues to radiate from the point of origin for the duration of the spell. Most emanations are cones or spheres.
A spread spell spreads out like a burst but can turn corners. You select the point of origin, and the spell spreads out a given distance in all directions. Figure the area the spell effect fills by taking into account any turns the spell effect takes.

The bolded part indicates that there can easily be tentacles underneath the stairs. In fact, every horizontal surface will sprout tentacles on its upper side. Since the area is a spread (which is a sphere), that includes horizontal surfaces that are layered omn top of each other. There need not exist a direct line between the spell's point of origin and every point from which tentacles can grow. However, there must exist an unobstructed path (which need not be a straight line), which cannot be longer than 20' in this case.
In 3.5, there are no individual tentacles to keep track of: there' just a non-quantifiable bunch of them, and they hinder movement, grapple and deal damage collectively.

 

[Greenfield]

The area for the Tentacles spell is the same as a Fireball: 20 foot radius burst. In a 30x30 room, it *can* fill the entire thing, if it's centered right.

There's a template for this in the back of the book, if you need a picture.

Presuming a 10 foot ceiling, the corners at the ceiling level will be clear, because of the way diagonals work in 3.5.

If you want to fill these areas, have it centered on something 5 feet off the floor, at the center of the space. It has to be anchored to a surface.

 

[Empirate]

It's in fact a spread, not a burst. Spreads go around corners, bursts do not. That's a difference which doesn't come into play that often, but there it is.

 

[frankthedm]

There's a template for this in the back of the book, if you need a picture.

And here are those areas showing how they differ from measured / based on the grid.




Here is some discussion on how 3.5 radii might work in 3D.



http://img339.imageshack.us/img339/3998/henticle.pdf
img339.imageshack.us/img339/3998/henticle.pdf

 

[Dandu]

So which do I use for Black Tentacles?

 

[frankthedm]

So which do I use for Black Tentacles?

20' radius. The PDF has two versions of tentacle templates because i wasn't sure which looked better