They're taking orders now and will begin shipping next week.
From the website:
The Thanatar-Calix class Siege-automata is a potent war machine and a wonder of the Mechanicum’s craft. Combining arcane and secret technologies, it carries a twin-linked mauler bolt cannon alongside a rare solex pattern heavy lascannon, whose beams of coherent light can reach far across a battlefield to lethal effect. The Thanatar-Calix's most destructive armament though is the graviton ram, a weapon whose origins are lost in the Dark Age of Technology. The ram's crushing waves of gravitational force can pulverise infantry and vehicles alike, and it can be utterly devastating at both range and close quarters.
The Thanatar frame, designed as a heavy weapons platform, is well armoured and further protected by atomantic shield generators, making it nearly impervious to all but the heaviest of weapons fire. Whilst it is slow to advance, the Thanatar-Calix’s durability allows it to close steadily on its target, moving ever nearer until its formidable weaponry can be brought into range.
The Thanatar-Calix may be taken in any Mechanicum army, including the Taghmata Omnissiah, Legio Cybernetica and Ordo Reductor army lists, in Horus Heresy games.
This is a complete, multi-part resin kit. It is available to pre-order today for despatch from Friday 17th April.