This robot is what happens when you combine together the know-how of some of the most secretive and advanced mechanicum factions in one lucid dream of destruction. Rare and in some quarters: sacred, these machines of destruction live up to their background on the tabletop!
There are some really nice weapons on the Thanatar, including a lascannon, the dreaded graviton ram, and a mauler cannon. All at BS4.
The WS is a bit lower though, and hence this machine is not quite as good as you might want it to be in close combat. Combined with only two attacks, the natural S=8 is a bit on the wasted side arguably. The equally high toughness combined with many wounds and its saves give this guy a huge survivability in the game. Combined with cybernetics resilience, not even poison is going to do too much damage to these machines.
One of the main problems with this Thanatar is what range to place him (her) at. Take for example the weapons. The lascannon has a long range and so can happily sit back and shoot away. The others do not have that luxury, so you are forced to move forward to make use of them. In doing so, you run the risk of getting tar-pitted (which is certainly what I might look to do against them in the game).
Although this unit is excellent against buildings, there is hardly any call for this in games as they exist at the moment. I look forward to seeing them deployed in cities of death though or boards with a large amount of buildings in close proximity to each other.
To be honest, I think I'd sooner spend points on other machines and robots overall, despite how survivable this particular machine actually is.
Thanatar-Calix (295 points).
The basic and naked build is totally viable in Horus Heresy.
Thanatar-Calix with Paragon of Metal (330 points).
Probably about as many points as you want to spend (unless you really want the searchlight and the enhanced targeting array). This can be a beast on the board. But I regard it as a huge points sink and think other army selections get a better return on their points.