I've installed this for 1.8.9 and it appears that the icosahedron is broken. In the inventory it appears with a missing model and texture (purple and black block place-holder) and when you place it into the world the model appears normally but fails to animate. The glow effect rises from the die but the die itself remains completely still.

The 1.8.9 version of this mod, just like many mods is not being pushed any longer. The developers of Forge and most other mods are making a push to use 1.10.2 as a version to work on. As far as the 1.8.9 branch of this mod fixing that particular issue, I'm afraid you're out of luck. The amount of work to devote fixing a mod in a minecraft version that has become irrelevant isn't, in my opinion, worth it.

Just to add onto what was said above. There actually is no way to get the D20 to animate due to a bug in forge in 1.8.9, which is no longer being developed.