Of unknown pedigree apart from their Hellex plasma mortar which is known to come from the Forge World of Ryza, the Thanatar are the heavy siege robots from the Mechanicum.
Their stat line is powerful to say the least. With both S and T pegged at 8 and W=4, these robots are not going to be removed off the board in a hurry. Its certainly worth getting the Paragon of Metal rule if the controlling player is only playing one of these beasts - coupled with the atomantic shielding, it will ensure it is that bit harder to take off the board for the enemy.
The Hellex weaponry is also notable for its AP2, S8 large blasts. And that's before we mention the re-rolling of cover saves. Sure, its not got the raw strength of a vindicator round, but it will cause problems for almost anything other than a flyer in the game.
Poor WS. Poor I. Not exactly the fastest robot in the factory. Don't get them in to combat as although they'll do okay, they're not fabulous. They need to rely on their shooting. But what did we expect for a siege robot? These are weaknesses, but they're totally in line with expectations. Perhaps the price tag (of the order of a land raider) is something of a put off, but these robots could turn the tide of a battle.
Thanatar, Paragon of Metal, Search Light, Enhanced Targeting Array (291 points)
The maxed out version of this robot with paragon of metal. Its fairly easy to leave this one alone and let it ply its trade on the battle field. Take out a terminator squad every turn!
5 Thanatars (1200 points)
Sure, they're the basic version. But there's five of them. FIVE. Hope you're pumping out at least four vindicator shots on target every turn. Because other than D weapons, instant death weapons, and primarchs, they're going to survive most other things. The only real disadvantage is that they're a unit. I'd suggest splitting them up if you have free force organisation chart slots.