From the developer:

Here’s how it works: A clay soldier is an item you can hold in your hand. It is crafted using a clay block (for substance) and soulsand (for sustaining intelligent life). Specifically, one block of clay and one block of soulsand can be used to create four soldiers. Once you have a nice little pile in your hands (up to 16), right-click to place them all at once. They will quickly spread out and begin to roam the landscape. They walk slow and don’t jump very high, but they have enough arm strength to climb up a single block. Unfortunately, since they lack a decent skeletal support structure (or even skin, for that matter), their delicate figures can be horribly mutilated by even the weakest of blows. A player can cripple a clay soldier in just a single punch. So naturally, they aren’t hostile towards anything bigger than themselves. So, what in the world are they qualified to fight against? …Other clay soldiers, of course!

This mod requires Manager Pack mod and Forge API.

Here’s a great fan-made trailer for the mod to give you an idea of what you can expect: