The Alpha Legion has many operatives, some of them mortal humans, and many of them their own. The Saboteur is a unique consul type available only to the Alpha Legion in the Age of Darkness and covers their own agents who are active on the battlefield.
From my point of view, I really like the background material for the Saboteur. He is supposed to be a master of assassination, covert operations and the destruction of enemy hardware. And as such, he is entirely fitting for the Alpha Legion to take in part of their army lists. Modelling wise, I'm not sure how I would approach these guys, but probably a "typical" Alpha Legion marine, but perhaps with cloaks and billowing robes to hide themselves inside of.
The first thing to note is that the Saboteur is not a compulsory HQ choice. That means we will still need a praetor, or named HQ like Armillus Dynat in preference to him. However, we could certainly have at least two saboteurs in addition to a compulsory HQ selection in a normal Age of Darkness force organisation chart. We'll take a look at whether two is enough in a moment. They are also hurt by never being able to join a unit - hence they might be an easy kill (and potentially contribute to a loss for the Alpha Legion through the Martial Hubris special rule).
Equipment wise, the saboteur comes with melta bombs and a cameleoline as standard, and therefore this will immediately lend itself to being a unit who will be sneaky, and be able to inflict damage on both buildings and units with an AV value. That said, they also have limitations such as never being able to be fielded in terminator armour (presumably because that's just not sneaky enough for these guys! And I can totally see why this would be true as well!). Hence the majority of the equipment that they're able to take is the light weight stuff, and not the heavy equipment such as boarding shields and bikes. This is entirely fair and resonates with the background for these characters.
Their main rules, however, are concerned with their deployment and how they enter play and their actions at the point of entry. Always starting in reserves, they arrive and then cause damage. When combined with some reserves manipulation, these guys could cause come serious damage when they come in to play at an opportune moment.
But let's look at that damage in a bit more detail.
There are two options. The first one is to cause an automatic penetrating hit. Now, unlike in previous editions, a tank is categorically not going to explode as a consequence of a single lucky hit. The best we can expect is an immobilisation, or, other kind of "lock down" for an enemy tank. That said, a lock down is a good result. Only being able to take snap shot, and potentially not being able to move is actually a really powerful result. But possibly, its simply not enough. Even if there are two saboteurs coming in to play for the Alpha Legion at some point during the game, the best that they might be able to do to a tank is to finish off the last Hull Point from the tank and thereby cause it to become a wreck. In this, it could be very useful. By that, I mean imagine that the enemy predator (or replace with whatever other tank you care to mention) has been reduced to their last hull point due to shooting in the previous turn. From this point of view, it would be a waste to use a full heavy support squad to shoot at it again in the next turn. The saboteur solves this problem in a very neat way -- an automatic hull point removed through a penetrating hit. This would free up the heavy support squad to target the next tank. In this way, the act of coming in from turn 2+ means that this is more likely to happen. Hence, it could be a good investment. That said, I would certainly prefer the option of the saboteur being able to kill a tank outright. Equally, I can see how that's simply not fun for the opponent. Yet, given the melta bombs, I could certainly envisage the saboteur outflanking on to the board and trying to finish the job that they started with the melta bombs.
On the other hand, they could elect to inflict d6 S6 hits on an opponent. Ignore the AP of these hits since the "typical" enemy will be a legion army (arguably), how many wounds is this likely to cause?
The average roll of a d6 is 3.5. Hence we have 3.5 hits per saboteur. The chances of causing a wound against a legion enemy is 2+ (i.e. S6 vs T4). This results in 2.92 wounds per saboteur. Of these, only 0.97 will get through (i.e. a failed power armour save). Hence on average, each saboteur is causing only a single wound on a typical legion enemy target. This is really not too terrific. And hence, I don't regard this as a good enough reason to take him. Only if there was an enemy all alone on the table - such as a character model down to their last wound, or a unit down to their last man - it might be worth it. But for me, this just isn't sufficient.
Below are a couple of possible builds for the saboteur.
Saboteur, combi-melta, artificer armour, power weapon, refractor field (130 points)
This is one who arrives on the board, follows up his penetrating hit with a melta shot, and then gets stuck in with close combat if possible (or with the melta bombs to finish a vehicle off).
Saboteur, artificer armour, refractor field, pair of lightning claws (130 points)
A bit more of an odd set up, but potentially one to take advantage of hitting a non-legion army with d6 S6 hits (e.g., Orks, Eldar, and those that have a 4+ save or worse, etc.) and follow up with a rapid assault with lightning claws. This has the potential to shine against these kind of opponents, but suffers from being slow to move.
I'm not 100% sold on the saboteur. I really want to like the character, but their effects are just too weak to justify their cost. The only way I could see to run him is against non-legion armies. There they could have a terrific effect against open-topped vehicles and those with 4+ or worse saves. Otherwise, I'm sorry to say, the saboteur is going to be staying at home. To improve the rules, I would have liked to have seen something similar to Tau's marker lights employed by the saboteur. i.e. they'd tagged a vehicle or person in advance and provide a bonus to hit, or a bonus on the vehicle damage table - something like that would make me much more tempted to take the saboteur. As they are, I'm just not going to take them outside of a very fluffy list.
See this combination for automatically blowing tanks up that involves the Saboteur. But don't expect it to not be FAQ'ed.