Horus Lupercal is the Primarch of The Sons of Horus, this means he is more or less the genetic template for the warriors of the legion, sharing his characteristics and genetic make up with them, in terms of primarchs Horus is as vanilla as it gets with his main abilities just being he is a competent military commander, and incredibly charismatic to both his primarch brothers and the mortals of the imperium, and a strange ability to command other legions just as efficiently as his own, playing to their strengths and having their warriors feel a sense of duty to him. Combined with being the first found primarch after the great scattering led him to be the emperor’s favoured son and eventually warmaster of the imperium after the emperor retired from the crusade to finish the webway project. But you don’t get a civil war named after you for being such a great loyal son, so eventually he betrayed the emperor due to his resentment towards the emperor and chaos corruption stoking his pride.
Horus is one of two primarchs with two different sculpts, one for his pre moloch form where he is still mostly a mortal man and then post moloch where he becomes the avatar of the chaos powers, so we will cover both these entries separately.

Horus Lupercal

Horus serves in game as primarily a command character, Costing 530 points, his biggest strength is his “master of war” ability, which is a simple +1 reaction per turn, this is super good as over the game it will give you 8-10 extra reactions as long as you keep Horus alive, which is not at all hard as he has eternal warrior two and most likely his justaerin bodyguard. His second ability, “sire of the sons of horus” allows you to gain extra prime slots if you run all your compulsory troops slots as either tacticals or despoilers. Oh no, I have to run the two best troops in the game in my army, how tragic that they’ll all be able to gain corrupted from the true believers prime benefit which is just mental, basically making status effects a non issue if you run them in large squads. Additionally regardless of your troops choices in the first turn when Horus is included in your army he will provide +1 inches to the movement of all Sons of Horus infantry and dreadnoughts in your army. This means turn one charges from assault marines or any other jump pack units are within your army and lets your other units move into position.
But wait, there’s more!, he’s also a strong duellist and melee threat, and unlike most primarchs who have slightly chump bodyguards like sanginous and his sanginary guard, or perturabo and his iron circle, Horus is packing the justaerin, who will casually murder anything they get into base to base with and make your opponent cry. He comes packing an AP3 S5 bolter with 5 shots in ranged and then in melee you can pick between the talon, or the world breaker mace. World breaker is a S10, Ap2 damage 3 crit 5+ strike at initiative 8, if you use finishing blow in a challenge he will casually throw out 4 or 5 damage strikes. That is enough to kill a regular praetor in a single strike, flatten any magos not on an abbeyant and make daemon sovereigns loose about half their health. The talon is only strength six, Ap2 damage one but g=has reaping blow 3, meaning if he is out numbered he will get 3 extra attacks bumping him to 9 attacks. he’s also weapon skill 8 which is in the bracket of higher weapon skill primarchs. Other than mortarion just being a slab of unkillable concrete, or archmagos Scoria being a blender on scorpion legs Horus is by far the best primarch in a head to head fight against an equal level hero. He will most likely need a spartan to transport him and his justaerin buddies as he has deep strike, but getting into melee from deepstrike takes two whole turns, while a spartan might have you in combat turn one. Overall a 10/10 primarch, only complaint is he’s slightly dry as opposed to some of the other primarchs with more flashy abilities
Horus Ascended

Horus Ascended is a more expensive, more powerful version of Horus who trades in some of his command prowess for just more damage dealing capabilities.
He retains his “Master of war” ability but replaces his “Sire of the Sons of Horus” with “spreading corruption”, this allows him to join malefic units, its okay but I don’t think you need a malefic bodyguard for a model with leadership 14 and cool 12. Where the real spice comes in is his eternal warrior 3, feel no pain 5+, toughness 8 and 8 wounds which basically means this guy is functionally unkillable by a conventional army. Like genuinely I cant think of any weapons that will destroy him quickly that isn’t titan scale weaponry, and even then a 2+ save, 4+ iinvuln and 5+ feel no pain while reducing incoming damage by 3 is gonna lead to him not going down without most an army dumping all its fire power into him.
And if you thought the last horus was scary, this guy is even more so with his cheeky weapon skill 9, highest in the game that isn’t mechanicum jank with 7 attacks and a strenght of 8, you just dont win challenges agaisnst him, he hit you with his fridge sized mace and you just kinda die, he smashes a land raider on a two I cant comunicate in text how dead this man makes you, what keeps him balanced is costing a cool 850 points, which means he is only legal in games bigger than 3400 points and most people wot play against him without an appropriate answer such as a titan, ordinatus, shadowsword etc.
Book Appearances
Unsurprisingly for a man with a 60 book long series named after him Horus turns up in a lot of the books, primarily the first three books in the Horus heresy and the last three end and death books feature Horus extensively.