It's pretty much a copy of Runehammer's terrain. I was coming up on an adventure through Avernus and happened across his video, so the timing was perfect.
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It's XPS foam (the pink stuff), black prime base, then a palate of red, orange, and yellow for painting. Glued together the small chunks with Gorilla glue. I only did a Modge Podge cover on one piece and decided I didn't need to do them all.
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The battlefield scatter is small chunks of foam, toothpicks, some old skeleton mini's, and a box of Citadel skulls from Amazon. Base is either foam board or chipboard. The ladders are craft sticks.
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The giant fangs are XPS foam, rough cut with a knife, then sanded down smooth and curved. XPS foam take really well to sandpaper.
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All the long gouges and marks are done by running a sharp blade. I used an Xacto knife.
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I really liked the way he did his grid - I did it the same way. Instead of cutting wide grooves that look like huge tiles, use a long ruler and score a line with the blade, putting a line every 1 inch. When the vertical and horizontal lines are done, go back with a Phillips head screwdriver and gently punch each intersection with a "+". This will give you a subtle but usable grid that doesn't take away from your terrain or palate.
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When we were playing the red fabric part was hot lava, the party had to navigate across it with an encounter on each one. After they reached the end and got the macguffin, a massive lava wyrm (cheap dollar storm Halloween snake painted to look like lava) surfaced, making all the platforms shift around and changing the battlefield each round.
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I also used these terrain boards for any encounter in general, arranging them differently and moving the scatter around.
This! and if you are lazy (like me) spray them bone white then a dark brown wash while still on the spru. Clip and touch up as needed.
https://www.amazon.com/Games-Workshop-99129999012-Citadel-Skulls/dp/B074WH9LX8
https://www.amazon.co.uk/Games-Workshop-99129999012-Citadel-Miniature/dp/B074WH9LX8
I bought mine from Bezos, I’m ashamed to say.
Got them at my local hobby shop, Citadel/Games Workshop makes them, they’re just the Citadel Skull box.
Here’s the Amazon link: https://www.amazon.com/dp/B074WH9LX8/ref=cm_sw_r_cp_api_fab_ScEHFbYZ1TYSV