My players just wrapped up a major arc and we had an interesting finale combat. I got to use some DIY 3d terrain I made out mostly out of styrofoam packing from my 3d printer, pin-stripe tape, posterboard and printed square grid paper from https://incompetech.com/graphpaper/. Here’s a view from the player side. But what I want to talk about is the yarn and the crystals.

The Pit Fiend at the top (who was successfully banished by a Dispel Good & Evil spell, more on that another day), was conducting a ritual that was amplified by the arrival of mortal souls (aka, the PCs). In channeling and trying to corrupt the energies of the cosmos, streams of energy flowed toward the Pit Fiend’s ritual site at the top. 

I marked the orange stream as the top of the initiative, and the blue stream has half-way through the initiative order. When it was a stream’s turn, I applied its effect and then I rolled a d8 to choose what kind of effect it would have the next time its turn came around:

1. Necrotic (Con)
2. Chaos (Cha)
3. Evil (Cha)
4. Life (N/A)
5. Law (Cha)
6. Neutrality (Cha)
7. Betrayal (Wis)
8. Good (Cha)

I would also roll a d12 to determine a clockface to which to orient the stream (i.e. if I roll a 2, it is pointed at 2:00). I think I learned to use d12 as a random direction from Seth Skorkowski’s video on critical hit/fumble tables. I assumed a certain symmetry. I oriented the battlefield so that 12:00 was directly away from my DM screen, so if I rolled a 6:00 I also considered that to be 12:00. Though I didn’t need to do this, the intent was to keep the streams relevant. 

At the start of the blue/orange Stream’s Turn, any creature between two orange / two blue pieces of yarn (the edge of the corresponding streams) has a different effect based on their alignment. If their alignment matches the stream (i.e. they are a devil and they are in a law or evil stream) then they are healed 2d10 HP. If they do not have the same alignment as the stream, they take 4d10 damage, or make the indicated save for half damage. The life stream healed all mortal creatures in the stream 4d10 HP. I used 4d10 from the trap table in the 5e DMG, which I use a lot for improvising damage.

The Pit Fiend, or any PC within 20 feet of a crystal, could use their bonus action to change the stream. In so doing, they could change the stream’s clockface up or down by 1 (i.e. 1:00 to 2:00, or 1:00 to 12:00), OR, change the effect up or down by 1 (i.e. shift the Evil stream to the Life stream).

It created some interesting movement and strategy for the players, trying to position themselves in life streams, only to have the Pit Fiend change it to an evil stream, and then having another player use their movement to change it back. You might have noticed some Demons that joined the fight too — at one point the PCs intentionally shifted a Chaos stream onto the Demons to prop up their HP so they could continue to soak damage! 

Some lessons learned:

• It was a good mystery for the PCs that I named the stream but they didn’t know exactly what would happen out of the gate. They were nervous about standing in the life stream the first time, but decided to risk it!
• Letting them change the stream was really engaging. There was a lot of discussion about that and a feeling like they were interacting with the Pit Fiend because he was also changing them. It is important that they have some time to get out of the way… like a telegraphed attack in a video game.
• It didn’t quite create as much movement as I anticipated.
• We definitely got sloppy in orienting to 1:00 or 2:00 by the end. It would have helped to clearly mark out the places the strings would go at the edge of the map, so that it could just be moved and placed there. 

Accordingly, the next time I do something like this, I think I might not require a bonus action to change the thing, but you might have to be adjacent to something to make the change.