For the fans of the Crimson Fists, the appearance of Alexis Polux in Extermination will no doubt be very exciting. Polux himself is described as Inwit-born (the home planet of Dorn himself) and a giant made of stone before even being elevated to the Space Marine Legions. As a marine, he towered above his battle brothers still. Through chance he came to command of Dorn's retribution fleet and proved his worth at the Battle of Phall and in many other battles beyond that. He is known to have been strong in close combat and a master of void based combat between spacecraft. And, of course, after the Heresy, he became the first master of the Crimson Fists Chapter.
Firstly, Polux allows one infantry squad to deep strike by teleportation on the the board. This could be a good move for a full squad of tactical marines to unleash the fury of the legion at BS5 (thanks to the Imperial Fists legion rules) for instance. Or perhaps a terminator squad?
In addition, he (and any unit he joins) can automatically pass or fail any morale or pinning test they're required to take during the game. This could be pivotal at the right moment, but is certainly quite situational within a given game.
The really interesting part of Polux profile is the ability to reduce his attacks in combat down to 1 in exchange for ignoring the unwieldy part of his power fist attack. Since the legion rules mean he must re-roll failed to hit rolls in personal challenges, this is not a huge negative to be fair. Indeed, it should probably be done more often than not as it is a real character slayer given that his natural strength is 5. To be clear: a power fist attack for S10 going at initiative is huge. Its instant death for most space marines where required (assuming they don't have an invulnerable save or eternal warrior).
As a warlord, he is a master tactician that permits redeployment of one unit as required. This is not so strong, as it could be highly situational.
For equipment, not only does he have his master crafted power fist (that could go at initiative), but also a Vigil shield for a great invulnerable save. Oh yes: he also has the usual grenades and a combi-melta gun to boot. Not bad at all.
Genuinely, the only weakness I can see is his standard (but void hardened) power armour. He does not have artificer armour. Hence only a 3+ save. I see this as being fluffy as Polux had command forced upon him and he stood up to the proverbial plate with superior ability. But nobody crafted him anything special and new armour was clearly hard to come by. He therefore needs to beware duelling against space marines who can sling a power sword better than he can his master crafted power fist. Don't get as arrogant as Eidolon and Polux will certainly work well!
Polux is a generic commander (and master of the legion) that can help one unit deep strike in to position. He is also a character killer, but not at the same level as Sigismund. To be clear, he's not going to go toe to toe with a primarch the way that Sigismund is capable of. But in a lower points game, he's going to be powerful.
Like many of his legion, he needs transport options. But perhaps he could deep strike off his own special rules on to the board? Indeed, this deep striking is an excellent way to build a larger unit around that might not ordinarily have the rule (think of a full plasma tactical support squad deep striking in to position for instance -- it reminds me a little of what Armillus Dynat is capable of pulling off, but he's not got Dynat's other rules to back it up as a force multiplier).
I therefore see him as an excellent upgraded Legion Praetor who can field a mixed or specialised force as required. He's well worth the points, and in lower points games could be a game winner if Sigismund is too expensive. But he's not the multiplier that Dynat is and a force should not be built around him -- treat him as you would a generic Praetor (an upgraded and great Praetor, but a Praetor none the less).