Ah, I thought that was a bit wierd. Still, first time trying the game...

 Assemble, even when activated from the strategic map, activates the DESTINATION AREA. This is the inner palace. No models may move from an activated area, precisely because it is activated. Since each area in the palace can only hold 3 units (and any number of heroes) any excess left in the inner-palace region would be removed from the game at a change of initiative phase. The rules that models can not be moved from activated areas takes precedence over the instruction on the Boarding Action card. You may not give any orders to a unit in an activated area.

What Assemble do you have ? Page 20 of the rulebook clearly shows Assemble with a Strategic Effect "Do not activate the destination area".

 Oh man! My card must be a misprint. It has no strategic effect!

To be fair, beaming up in to the catacombs early on would be a mistake anyway. Assaulting Horus (considering his command centre is a fortified area) is difficult. Couple with that a coexistence battle and an obvious Port Landing move by the Chaos player once the initiative shifts and some of your excess units die and the fact that any experienced player will stock up on at least one hero and more units in the Vengeful Spirit to guard against such a move (Mortarion is particularly nifty) and it's a bold move, a big risk and a potential game-ender now you've left the palace under-defended or got the Emperor injured or killed.

Horus Heresy is a lot like chess really. Over-commiting yourself rashly is often a mistake. If you know for sure the Chaos player has used all the Port Landing cards in his hand and he's left Horus vulnerable, go for it. Otherwise, weigh up whether it is worth the risk and whether you could afford to wait out the Imperial Victory.

Zerodemon said:

Things is, I've never seen Horus survive even in the Command Center if/when attacked by the Imperials. Even the math required for him to do so seems to give him a very long shot at best (personally I think it's impossible). Imperials bring in 3x each of Custodes, Blood Angels and Imperial Fists, with Emp, Dorn and Sangy. Dorn pumps all Imp units to Rank IV 9x4/2 = 18 cards. With the Emperor's ability go through the combat deck, you only need to discard until you'r guaranteed to draw "Driven Before the Storm". That one card routs two units, no cancelling possible, so now Horus is down to 1 Rank III unit and himself for two more iterations. Worse, Sanguinius + 3 Blood Angels effectively negate fortifications (for +3 dmg), not to mention all those direct damage to hero cards. However, even that's not the worst part. Imperials only need to kill that one Rank III unit in the Command Center and they can move in, so on top the Assault, Horus has to stand on his own against 8 iterations of coexistance battle. That's just not happening.

 Seems broken for sure. Possibly Boarding Action should activate the landing area too.

Oh wait. There are many assumptions here that I'm not really considering. This is a VERY specific set of moves.
Firstly, you're assuming that the Traitor player doesn't bury any of your orders (I would if you strategic mapped on the Vengeful Spirit.)
Secondly, you're assuming the traitor player making a 3 point move early in the game, which, any player will tell you, is a terrible move to make.
Thirdly, You're assuming one of those moves wouldn't involve reinforcing the Vengeful Spirit with an extra Hero and a couple more units.
Finally, forgoing the idea of burying an order, what's to prevent the traitor player placing an order on top of one of yours.

This is a great move to play against a first time player if you're experienced (and a bit of a jerk, haha.) but against most people, it just wouldn't work because it is too obvious. 

Zerodemon said:

This point is the only one that doesn't work. Command Center is a fortified area, so limit of 3 units (which it starts with). Extra Hero doesn't protect Horus, since the Imperial Player (unless he's braindead), will pour all the damage on Horus.

 Incredibly valid, but I'm thinking more about Strategic Effects. Mortarions is pretty nifty in this very specific situation.

Pulling out Morty from Terra I feel would leave the Traitors too weak on the ground and risk the Imperials getting a Space Port Victory. Or even if not Space Port Victory, then allowing the Imperials to take back the north-west space port where Mortarion starts at (in scenario 1). If the Imperials hold both northern ports, Traitors have to secure the southern ones, move up north and take both ports that the Imperials have had time to reinforce, which could lead to a Holdout Victory.

I like the things as they stand. As Traitor, you have to get a feel for when the Imperials are ready to launch their attack against Horus and bring him down to Terra or mess up with the Imperial plan using your own order cards, forcing them to take bury actions.

Back on topic, it's all moot anyway. The answer is no, this combination is not too powerful.

Yeah, I agree.  It is not too powerful.  In my opinion, the traitor should hardly ever move the initiative more than two spaces in his turn anyway (to avoid stuff like that).  Another valid option is port landing or drop pods to put Horus onto the map.  Once Horus is on the map, especially if he's far away from the action, it's very unlikely that he'll die.  Especially in cases of the scenarios that give you ways to lower the cost of actions, it is a very good idea to just drop Horus to Terra immediately.  Anyway, like I said, it's not like that's impossible to counter or plan ahead for.

Joeyman said:

Sky Fortress + Titans Stride the Earth can really put a damper on Horus's plans, SF brings the pain, a huge mass of Imperials, most likely forcing Horus to retreat, TStE Event follows (gotta work the Initiative Track naturally), allowing a free attack against Horus with another big mass, this time Horus can't retreat anymore (unless you've passed a Refresh Phase too).

True, but a later death beats a sooner one. 

Joeyman said:

I don't know, if you're going to lose, might as well lose early . But that's only for the losers .