2023 Advent Calendar Day 1: Sela's Slippery Sidewalk (spell)

 Last year, my sister got me the official WotC D&D advent calendar as a Christmas present. Which was very nice of her, but I couldn't help but be disappointed in the contents of the gift itself - all of it was branded merch, and none of it was really useful for running or playing the game. A pencil and a pack of sticky notes, I guess, but I was hoping for real content: monsters, spells, character options, maybe an adventure or two to round things out.

So this year I decided to make that myself. Every day of December through Christmas Eve, I'll be posting one piece of useful* winter-themed homebrew content for 5th edition. Most of it will be DM stuff like monsters, but some will be player options. Almost none of it will be playtested.

(I definitely didn't spend 11 months thinking about this, and then forget to post on the very first day... It's time zones, I tell you. Blame it on the time zones.)

Sela's Slippery Sidewalk

Conjuration cantrip, 1 action, 60 feet, VS, Concentration up to 1 minute

You create a slippery puddle of ice on the ground in a 5-foot square you can see within range. Any creature in the ice's space when you cast the spell must succeed on a Dexterity saving throw or take 1d4 cold damage and fall prone. A creature must also make the saving throw when it moves into the ice's space for the first time on a turn or ends its turn there.

The spell's damage increases by 1d4 when you reach 5th level (2d4), 11th level (3d4), and 17th level (4d4).

The create bonfire cantrip occupies a really unique corner of the design space, as the only damage cantrip with an ongoing concentration effect, as well as an area of effect. This has a similar mechanic, but trades damage for the debuff of falling prone. It also has a similar effect to grease, a 1st-level spell, but grease covers a much larger (10' square) area and, critically, doesn't require concentration, so with those tradeoffs I think a cantrip is appropriate.

Previous iterations had no damage at all, since prone is a fairly strong debuff. But it's also easy to just stand back up, likely on the same turn, so a little damage is also appropriate. If only for the mental picture of monsters falling on their butts so hard they die as they try to chase after you. And having slipped and fallen on many iced-over sidewalks, I can guarantee that it hurts.

* "useful" means content & mechanics you can put in your game right away. I'm not guaranteeing any of it will actually be useful.