Weapons drawn, you round the corner ahead, sticking to the shadows to avoid detection. Your plan is foolproof: sneak in, take them by surprise, and...

...then your DM rolls a natural 20. You've been spotted! A potent blast of fire comes streaking out of the darkness beyond, incinerating your sturdy barbarian. The cleric sprouts an arrow from their chest and goes down. You hear shouts and the dread clank of metal behind you. No hope now: your wizard isn't powerful enough to effect a magical escape, and half your party's busy pushing up daisies. The shouts come closer, and your assailants burst into view with steel drawn, a malevolent glint in their eyes...

If this scenario sounds uncomfortably like your last D&D session, fear no more! We have the solution for you: a custom-designed d10 cleverly disguised as a d20.

Step 1: OpenSCAD Model File and Directions

You'll need to download and install the newest version of OpenSCAD, which has support for text.

Once you've installed OpenSCAD:

open it and load the attached SCAD file;
press F6 to render;
export as STL (File -> Export -> Export as STL).

Now you have a 3D model of the Anti-Crit d20 in STL format.

From here, you have two main options:

If you have access to a 3D printer, consult the manual or online documentation for instructions on converting STLs to gcode and printing.
If not, upload the STL to Shapeways and pay them to print it. (Yes, this costs money - but it also means you have a wider range of materials to choose from!

The SCAD file is commented - if you're curious about the mathematics of crafting a d20, or if you want to tweak it to create a normal d20, read the source! (We're also happy to explain anything in there, so please ask questions!)