So I know in some books(Eisenhorn), there is the ability to bind daemons. And I know that there is nothing regarding this in any of the rule books, but if this were to be possible how could it be implemented? It would be incredibly difficult and will most likely kill the character but just curious.

Eisenhorn made Daemonhosts. Basic rules for that in the original Dark Heresy Corebook and more info in Disciples of the Dark Gods.

Daemon binding is mostly a Dark Heresy thing but I'll bet there's Black Crusade rules for it too. Don't expect to be loved, liked, tolerated or even not shot on sight if the Inquisition gets wind though. BTW they WILL get wind.

Well I would expect nothing less from the inquisition, or anybody for that matter. But thanks I'll have to take a look at that.

I would assume he's probably asking from a GM standpoint rather than that of a player (because no sane player would even think of it unless the whole party has sided with Karrad Val), but I could be wrong.

But yes, as was said above, DH has many rules for it, and BC even adds rules for possession, updated daemon weapons, and a slew of other things daemonic (such as unique rules for psychic powers). As all the games are (more or less) broadly compatible, those are your places to start looking when it comes to official rules.

Two related examples from Rogue Trader campaigns I've played in (if maybe not exactly what you looked for):

1. In one campaign, the astropath transcendant PC made a Dark Pact (see Dark Heresy corebook) with a daemon for more power. In return, "all" he had to do was get crew to worship the daemon. So effectively he formed a daemon cult in the underdecks. Leading to the rest of the party hunting him down, and him dragged into the warp by said daemon to escape. Returning many sessions later as a horrendously powerful possessed psyker.

2. In another campaign, the party defeated a corrupted rogue trader. While searching the fallen rogue traders' quarters, the navigator PC found a tome. Which turned out to be the Book of Lorgar, which he eagerly read. The GM warned the navigator's player that with the insanity and corruption his character would gain from learning sorcery and rituals from the book, he'd be unplayable or dead within a few sessions. Which turned out to be true, as we had to gun down the navigator and barter the coverup with his navigator house for favours.

Any PC reading chaos tome or taking instruction from a daemon or chaos worshipper should realistically gain insanity and even corruption points, for accepting the ways of the warp. Perhaps modified by Willpower rolls or Willpower bonus. Actually learning sorcery has even more CP gain as per Edge of the Abyss rules.

Unsurprisingly, the rules to summon, master and bind daemons to animate and inanimate objects alike are presented in Black Crusade.

Yes, you can try creating daemonhosts (or turn yourself into one, or play as one with Tome of Decay rules), binding daemons into trinkets, jewellery, weapons, tanks, fighter jets or even your own ship. If you are particulary unlucky, your amateurish ritual will actually succeed, but no matter the result, doing this is essentially saying goodbye to sanity of mind, purity of soul, and, much moreimportantly, the thickness of your wallet. Yes, you might not care that Inquisition has put your entire family to the torch due to your excommunication, but they will arrest your bank accounts and trade enterprises too.

I was thinking that it could be cool if the players were to be given a situation where they could be given the power to control a daemondhost. The players wouldn't know that it was a daemon, but perhaps like in the soul drinker books the daemon would be disguised as something more holy like. It will be a good challenge to see if the players can see through their greed and figure out that something is off.

A hidden daemonhost is definitely possible. Chaos can be a tricksy beast and subtle at times.

Rulewise, daemonhosts in DH corebook can appear completely normal. In BC, there is a gift of the gods that makes the character appear totally normal (it's a psychic effect which can be seen through but it's difficult).

Unsurprisingly, the rules to summon, master and bind daemons to animate and inanimate objects alike are presented in Black Crusade.

Also in one of the DH books, are rules for summoning daemons and asking "favors" of them. How long they stay, what they do, and who exactly you summoned, it as GM discretion.

On it's most basic level, the binding of a daemon would be a ritual. The rules for constructing such are found in the BC core book.

A hidden daemonhost is definitely possible. Chaos can be a tricksy beast and subtle at times.

Rulewise, daemonhosts in DH corebook can appear completely normal. In BC, there is a gift of the gods that makes the character appear totally normal (it's a psychic effect which can be seen through but it's difficult).

Blood Angels Omnibus

Yep, Chaos can put many weird hats on their operatives.