The second half of Liber Questoris covers the Legio Titanicus - the Titan Legions of the Horus Heresy.
Without doubt, the Titans of the Forge Worlds were responsible for more of the Great Crusade victories than the Astartes themselves. They are deadly. They are big. And there's lots of special rules about them.
Detachments.
You can take a titan with an ordinary army as a Lords of War choice. Or you can use the special Titan Ordinal primary detachment. You only get 1 titan though, and a bunch of Secutarii and their transports. I really wanted Titan maniples against each other. Of course, that's still possible, you just have multiple single detachments. But its a bit odd.
Profiles.
We have (typically) 4 profiles per titan. One for each region of their body: Head, Carapace, Arms, and Legs. They each have different armour values and hull points.
Speaking of armour values, there's 2 versions: primary armour (which you will use all the time) and exposed armour (once you get to crippled status for a titan). And you also have void shields to get through first which are AV=13 and can be re-booted depending on the crew trait of the titan. War horns are also mounted on titans which can unleash terrible noises. This reduces advanced characteristics by (X) within 24 inches which is handy for forcing routs and similar.
Damage.
Damage adds up as you might expect by reducing hull points through penetrating and glancing hits. Glancing hits though result in a roll on the damage table but if crippled then you get 1 hull point instead. The damage table gives some negative modifiers to your next turn. But if you get down to zero hull points, the relevant part (head, carapace, arms, and legs) suffer significant problems and their armour counts as exposed. Crippled parts that are hit again roll on the critical damage table. It is here you get mortal blows to the Princeps, the reactor exploding, weapon breakages, and for the legs - the titan literally falling over (splat!).
Tactics of Desperation.
Small units have the option of swarming the titan as an assault action. They literally climb the structure looking for minor flaws. This involves an intelligence test with modifiers based on the units type and the titan's crew. If successful, then the attacking unit can make a choice of effects to cause the titan such as stripping the armour (future attacks are against the exposed armour value for the turn) and jamming the actuators (the controlling player can't move the titan in the next turn). The swarm attack can also be to cause damage (get bonuses on the intelligence test) or to escape harm (reduce incoming damage in exchange). Of course, the mitigation cost is steep. S=8 time d6+1 incoming hits result against the unit, but if a cool check is passed, then these are only at 1 damage and AP=4. If failed, then 2 damage and AP=2.
Engine Kill and Missions.
The missions included in the book make clear that you should tell your opponent you will be fielding a titan, and that you can't take the titan detachment outside of these special missions. That's okay.
Different types of titan (Scout like warhouds, or Battle like reavers) are worth different victory points. But what I found really interesting with the missions is the existence of strategic objectives that only the titans can take. There are macro emplacements (AV=15, gains 5 victory points per 5 hull points lost - with an infinite number of hull points possible curiously enough! I've not seen an infinity in a game yet - this is not a great idea gaming wise I feel); communications bunker (AV=14, 20 hull points) which is worth 10 victory points and reduces your enemy reserve rolls to 6+ for the rest of the game (I like this); and a Command Post (same as the above in terms of stats) worth 10 victory points which causes suppressed for every unit in their army if they fail a cool check outside of combat.
