Background.
I mean - yes. He will kill your own men. But this does have a place in some circumstances.
The Sons of Horus are very similar to how they appeared in the first edition, even down to the names of some their special rules. Of course, they are the arch traitors, but there are some in their number who remember well the Luna Wolves of old and cling to the ideals of the Emperor's dream still. I am very glad to see such elements being acknowledged formally in almost every legion detailed in the second edition.
Legion Rules Review.
Merciless Fighters. Retaining the same name as first edition, the core special rule for the Sons of Horus is to reduce by -1S any incoming attacks in melee. This is a great rule and represents a near peerless advantage within close combat. Consider for a moment power fists. They are incoming at S=7, and therefore lose the instant death dealing against marines. Your fighters that have more than one wound will be laughing to say the least. This is incredible really. Naturally, the legion will favour such melee based tactics (take the Reavers- you know you want to!), but is not a slouch in the vehicle department either since this special rule also provides a bonus to ramming actions. Overall then the Sons of Horus are mainly a close combat legion, but don't be surprised by plenty of variation. I long remember a Sons' drop pod assault army I once faced off against backed up by long range tank firepower which was an excellent blend of treats to take care of.
Their special upgrade for legion centurion is the Dark Emissary which I intend to review distinctly. Alongside this, they gain access to The Warmaster's Armour, and select traits for the warlord from The Warmaster's Own.
Advanced Reaction.
Death Dealers allows a reaction to being targeted by incoming firepower in the opponent's turn. If triggered and used as a reaction, the nominated unit fires back at +1BS. This then is a strong reaction, but as with other legions the temptation is to horde this until a vital moment. I suggest otherwise: use it as soon as you can since you will find your legion wanting to get into combat and therefore you may not have a chance to play it otherwise.
Difference to First Edition:
Neutral. (On balance, the new rules provide about the same level of competitiveness).
Overall, the Sons of Horus have seen changes compared to first edition. The old Merciless Fighters rule worked differently and allowed bonus attacks. Here we see a reduction in strength of incoming attacks instead which will be amazing and an absolute increase in power levels for the legion. Missing here is the Edge of the Spear rule which will be a loss for deep striking armies - I know this will hurt many players out there who like their terminators and this I feel is why on balance I'm approaching neutral rather than a power increase difference level. The loss of Bitter Pride probably won't affect too many players though. Arguably important to note here is that the former need for large blobs of melee units is now gone - the Sons of Horus can function well with minimum sized squads thanks to the new implementation of Merciless Fighters. Hence this is why on balance I think the rules are still strong, but on balance probably about the same power level as first, if rather different overall.