Falling after being Knocked Prone on a Slope

Question

I'm setting up a trap for my players.

They will be entering a tunnel with a rather steep section - steep enough to require climb checks to avoid falling but not so steep it can't be counted as "ground".

Perhaps like 60º-70º?

My BBEG (Druid/Wizard) will have trapped this slope using two Glyphs of Warding. He stores Grease on one, and buries it using Mold Earth more than 1 foot deep in the stone. He then stores Spike Growth on the other (burying it as well).

When the PCs come within 5ft of the Spike Growth, it will trigger. The Grease will trigger one round after the Spike Growth, hopefully knocking the PCs prone.

I know that Flying creatures being knocked prone fall. I imagine that someone being knocked prone on a 60º incline would also fall, but might have a chance to catch themselves.

My question is how do I rule being knocked prone while on a steep floor?

My options I'm considering are:

1. The PCs fall until something physically stops them (like falling if flying)
2. The PCs get a chance to grab onto a surface each 5' of movement they fall.
3. The PCs get a chance to grab onto a surface once/round they fall. Each \$X\$ they succeed the check by translates to \$(X + 1) \times 5\$ feet less they fall.

Of course, each 5' they move over the steep floor covered by Spike Growth, they'll take 2d4 piercing damage. #evil_dm_laugh

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As an aside, my PCs have already encountered several Glyphs of Warding, so this won't come as a huge surprise for them.

Answer 1

Tomb of Horrors has a somewhat similar scenario

The Tomb of Horrors module in Tales from the Yawning Portal includes the "Locked Oaken Door" scenario (p. 219-220), which reads as follows:

When one or more characters move there, the floor beyond the door will begin to tilt downward, with the north end slowly sinking. If this occurs, quickly state how the floor is beginning to slant, and have the characters roll initiative. On initiative count 10, all characters north of the door fall prone and slide 10 feet to the north.

A character can attempt to scramble back up the ramp to the south by making a successful DC 10 Strength (Athletics) check, or two successful checks if using the Dash action. Those who succeed still slide 10 feet north on initiative count 10 in each round, but they can also climb upward according to the normal rules for doing so. A character whose check fails by 5 or more slides an extra 5 feet to the north. [...]

Here, the PCs would usually try to ascend the slope each turn, but should they fail significantly they actually fall further down the slope. I like this mechanic and have used it for slopes in my own games, changing the DC and similar things for a given situation.

One thing to remember is that a slope is not very much of a challenge for somebody who can teleport or fly, a flying creature can stand from prone and then fly, and a magic user can simply cast misty step to get away from the slope almost entirely.