Given a particle emission, how do you make a null (or some other 3d object, for that matter) follow a specific particle?

1. Select the null.
2. In the ICE toolbar, click Kinematics > Effects > Transform Objects by Particles.
3. Pick the point cloud.

The null is now constrained to particle 0. There’s an ICETree on the null where you can change the particle index, to constrain the null to a particle with a different ID.

For a more detailed, low-level look into this, see the video ICE Kine – Drive from particles by Eric T.