Forge World is taking orders for a pair of new Mechanicum units over in their webshop.
From the announcement:
The Mechanicum Thallax Cohort with Multi-melta set and The Mechanicum Thallax Cohort with Photon Thruster sets, designed by Will Hayes, contain three resin models and is available to pre-order now. These sets will be despatched from Monday 31st March. Full rules for these models can be found in The Horus Heresy Book Two: Massacre.